Dual electrode system for a continuous analyte sensor

ABSTRACT

Disclosed herein are systems and methods for a continuous analyte sensor, such as a continuous glucose sensor. One such system utilizes first and second working electrodes to measure analyte or non-analyte related signal, both of which electrode include an interference domain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 11/692,154filed Mar. 27, 2007 now U.S. Pat. No. 7,761,130, which is acontinuation-in-part of U.S. application Ser. No. 11/543,539 filed Oct.4, 2006 now U.S. Pat. No. 7,467,003, which is a continuation-in-part ofU.S. application Ser. No. 11/004,561 filed Dec. 3, 2004 now U.S. Pat.No. 7,715,893, which claims the benefit of U.S. Provisional ApplicationNo. 60/527,323 filed Dec. 5, 2003, U.S. Provisional Application No.60/587,787 filed Jul. 13, 2004, and U.S. Provisional Application No.60/614,683 filed Sep. 30, 2004. Each of the aforementioned applicationsis incorporated by reference herein in its entirety, and each is herebyexpressly made a part of this specification.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods formeasuring an analyte concentration in a host.

BACKGROUND OF THE INVENTION

Diabetes mellitus is a disorder in which the pancreas cannot createsufficient insulin (Type I or insulin dependent) and/or in which insulinis not effective (Type 2 or non-insulin dependent). In the diabeticstate, the victim suffers from high blood sugar, which may cause anarray of physiological derangements (for example, kidney failure, skinulcers, or bleeding into the vitreous of the eye) associated with thedeterioration of small blood vessels. A hypoglycemic reaction (low bloodsugar) may be induced by an inadvertent overdose of insulin, or after anormal dose of insulin or glucose-lowering agent accompanied byextraordinary exercise or insufficient food intake.

Conventionally, a diabetic person carries a self-monitoring bloodglucose (SMBG) monitor, which typically comprises uncomfortable fingerpricking methods. Due to the lack of comfort and convenience, a diabeticwill normally only measure his or her glucose level two to four timesper day. Unfortunately, these time intervals are so far spread apartthat the diabetic will likely find out too late, sometimes incurringdangerous side effects, of a hyper- or hypo-glycemic condition. In fact,it is not only unlikely that a diabetic will take a timely SMBG value,but the diabetic will not know if their blood glucose value is going up(higher) or down (lower) based on conventional methods, inhibiting theirability to make educated insulin therapy decisions.

SUMMARY OF THE INVENTION

A variety of continuous glucose sensors have been developed fordetecting and/or quantifying glucose concentration in a host. Thesesensors have typically required one or more blood glucose measurements,or the like, from which to calibrate the continuous glucose sensor tocalculate the relationship between the current output of the sensor andblood glucose measurements, to provide meaningful values to a patient ordoctor. Unfortunately, continuous glucose sensors are conventionallyalso sensitive to non-glucose related changes in the baseline currentand sensitivity over time, for example, due to changes in a host'smetabolism, maturation of the tissue at the biointerface of the sensor,interfering species which cause a measurable increase or decrease in thesignal, or the like. Therefore, in addition to initial calibration,continuous glucose sensors should be responsive to baseline and/orsensitivity changes over time, which requires recalibration of thesensor. Consequently, users of continuous glucose sensors have typicallybeen required to obtain numerous blood glucose measurements daily and/orweekly in order to maintain calibration of the sensor over time.

The preferred embodiments provide improved calibration techniques thatutilize electrode systems and signal processing that providesmeasurements useful in simplifying and updating calibration that allowsthe patient increased convenience (for example, by requiring fewerreference glucose values) and confidence (for example, by increasingaccuracy of the device).

In a first aspect, an analyte sensor configured for measuring an analytein a host is provided, the sensor comprising: a first working electrodedisposed beneath an active enzymatic portion of a sensor membrane; and asecond working electrode disposed beneath an inactive-enzymatic portionof a sensor membrane or a non-enzymatic portion of a sensor membrane,wherein the sensor membrane comprises an interference domain locatedover the first working electrode and the second working electrode,wherein the interference domain is configured to substantially blockflow of at least one interfering species.

In an embodiment of the first aspect, the interference domain isconfigured to substantially block at least one interferent selected fromthe group consisting of acetaminophen, ascorbic acid, bilirubin,cholesterol, creatinine, dopamine, ephedrine, ibuprofen, L-dopa,methyldopa, salicylate, tetracycline, tolazamide, tolbutamide,triglycerides, and uric acid.

In an embodiment of the first aspect, the interference domain isconfigured to substantially block at least one interferent selected fromthe group consisting of hydrogen peroxide, reactive oxygen species, andreactive nitrogen species.

In an embodiment of the first aspect, the interference domain isconfigured to substantially block at least one non-constant noisecausing interferent.

In an embodiment of the first aspect, the interference domain comprisesan auxiliary electrode comprising a conductive material, wherein theauxiliary electrode is configured to modify an electrochemicalinterferant such that the electrochemical interferent is renderedsubstantially electrochemically non-reactive at the working electrode

In an embodiment of the first aspect, the auxiliary electrode comprisesa form selected from the group consisting of a mesh, a grid, and aplurality of spaced wires.

In an embodiment of the first aspect, the auxiliary electrode comprisesa polymer, wherein the polymer comprises a material that is permeable toan electrochemical interferant.

In an embodiment of the first aspect, the interference domain comprisesa blend of at least one hydrophilic component and at least onehydrophobic component, wherein the interference domain is configuredsuch that the sensor provides an equivalent analyte signal response toat least one interferent that does not substantially affect accuracy ofan in vivo analyte concentration measurement, and wherein the sensor isconfigured to provide a linear response to analyte concentration, invivo within in a physiological range.

In an embodiment of the first aspect, an amount of the hydrophobiccomponent is greater than an amount of the hydrophilic component.

In an embodiment of the first aspect, the blend of at least onehydrophilic component and at least one hydrophobic component comprisesat least one hydrophilic substituent of a polymer and at least onehydrophobic substituent of a polymer.

In an embodiment of the first aspect, the hydrophilic component and thehydrophobic component each comprise at least one cellulosic derivative.

In an embodiment of the first aspect, the cellulosic derivativecomprises at least one of cellulose acetate and cellulose acetatebutyrate.

In an embodiment of the first aspect, the interference domain comprisesa silicone material configured to allow transport of an analytetherethrough.

In an embodiment of the first aspect, the silicone material comprises ablend of a silicone elastomer and a hydrophilic copolymer.

In an embodiment of the first aspect, the hydrophilic copolymercomprises hydroxy substituents.

In an embodiment of the first aspect, the hydrophilic copolymercomprises a PLURONIC® polymer.

In an embodiment of the first aspect, the silicone material has amicellar jacket structure.

In an embodiment of the first aspect, the interference domain comprisesa polyurethane.

In an embodiment of the first aspect, the interference domain comprisesa polymer having pendant ionic groups.

In an embodiment of the first aspect, the interference domain comprisesa polymer membrane having a predetermined pore size that restrictsdiffusion of high molecular weight species.

In an embodiment of the first aspect, the high molecular weight speciescomprise at least one of glucose and ascorbic acid.

In an embodiment of the first aspect, the sensor is configured to besubcutaneously implanted.

In an embodiment of the first aspect, the sensor is configured to beintravascularly implanted.

In an embodiment of the first aspect, the sensor comprises anarchitecture with at least one dimension less than about 1 mm.

In an embodiment of the first aspect, the interference domain isconfigured to substantially block passage therethrough of at least oneinterferent such that an equivalent glucose signal response of theinterferent is less than about 60 mg/dl.

In an embodiment of the first aspect, an equivalent glucose signalresponse of the interferent is less than about 30 mg/dL.

In an embodiment of the first aspect, the equivalent glucose signalresponse of the interferent is less than about 10 mg/dL.

In an embodiment of the first aspect, the membrane comprises at leastone compound selected from the group consisting of Nafion, sulfonatedpolyether sulfone, polyamino-phenol and polypyrrole.

In an embodiment of the first aspect, the membrane comprises at leastone enzyme configured to metabolize at least one interferent, whereinthe enzyme is selected from the group consisting of a peroxidase and anoxidase.

In an embodiment of the first aspect, the interference domain comprisesa sorbent having an affinity for an interfering species.

In a second aspect, an analyte sensor configured for measuring glucosein a host is provided, the sensor comprising: a first working electrodeconfigured to generate a first signal indicative of glucose andnon-glucose related electroactive compounds having a first oxidationpotential; and a second working electrode configured to generate asecond signal indicative of non-glucose related electroactive compoundshaving the first oxidation potential; and electronics configured toprocess the first signal and the second signal, wherein the sensorfurther comprises at least two mechanisms configured to substantiallyblock or substantially eliminate noise in the sensor signal, themechanisms comprising a first mechanism disposed on the sensor andconfigured to reduce or substantially block interferants from reachingthe first working electrode and the second working electrode, and asecond mechanism in the electronics comprising programming configured toprocess the first signal to substantially eliminate the signalassociated with the non-glucose related electroactive compoundstherefrom.

In an embodiment of the second aspect, the first mechanism comprises aninterference domain.

In an embodiment of the second aspect, the first mechanism comprises amechanism configured to increase flow around at least a portion of thesensor.

In an embodiment of the second aspect, the first mechanism comprises aphysical spacer.

In an embodiment of the second aspect, the first mechanism comprises atleast one mechanism selected from the group consisting of a hydrogel, ascavenging agent, a bioactive agent, a shedding layer, and aninterferent scavenger.

In an embodiment of the second aspect, the first mechanism comprises anauxiliary electrode configured to electrochemically modifyelectrochemical interferants to render them substantiallynon-electroactively reactive at the first working electrode and thesecond working electrode.

In an embodiment of the second aspect, the non-glucose relatedelectroactive compounds having a first oxidation potential comprisenon-constant non-glucose related compounds.

In a third aspect, a method for providing a substantially noise-freesignal for a glucose sensor implanted in a host is provided, the methodcomprising: implanting a glucose sensor in a host, the glucose sensorcomprising: a first working electrode disposed beneath an activeenzymatic portion of a sensor membrane; and a second working electrodedisposed beneath an inactive-enzymatic or a non-enzymatic portion of asensor membrane, wherein the sensor is configured to substantially blockone or more interferants from reaching the first working electrode andthe second working electrode; generating a first signal indicative ofglucose and non-glucose related electroactive compounds having a firstoxidation potential; generating a second signal indicative ofnon-glucose related electroactive compounds having the first oxidationpotential; and processing the first signal to substantially eliminatethe signal associated with the non-glucose related electroactivecompounds therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a continuous analyte sensor, includingan implantable body with a membrane system disposed thereon

FIG. 1B is an expanded view of an alternative embodiment of a continuousanalyte sensor, illustrating the in vivo portion of the sensor.

FIG. 1C is an expanded view of another alternative embodiment of acontinuous analyte sensor, illustrating the in vivo portion of thesensor.

FIG. 2A is a schematic view of a membrane system in one embodiment,configured for deposition over the electroactive surfaces of the analytesensor of FIG. 1A.

FIG. 2B is a schematic view of a membrane system in an alternativeembodiment, configured for deposition over the electroactive surfaces ofthe analyte sensor of FIG. 1B.

FIG. 3A which is a cross-sectional exploded schematic view of a sensingregion of a continuous glucose sensor in one embodiment wherein anactive enzyme of an enzyme domain is positioned only over theglucose-measuring working electrode.

FIG. 3B is a cross-sectional exploded schematic view of a sensing regionof a continuous glucose sensor in another embodiment, wherein an activeportion of the enzyme within the enzyme domain positioned over theauxiliary working electrode has been deactivated.

FIG. 4 is a block diagram that illustrates continuous glucose sensorelectronics in one embodiment.

FIG. 5 is a drawing of a receiver for the continuous glucose sensor inone embodiment.

FIG. 6 is a block diagram of the receiver electronics in one embodiment.

FIG. 7A1 is a schematic of one embodiment of a coaxial sensor havingaxis A-A.

FIG. 7A2 is a cross-section of the sensor shown in FIG. 7A1.

FIG. 7B is a schematic of another embodiment of a coaxial sensor.

FIG. 7C is a schematic of one embodiment of a sensor having threeelectrodes.

FIG. 7D is a schematic of one embodiment of a sensor having sevenelectrodes.

FIG. 7E is a schematic of one embodiment of a sensor having two pairs ofelectrodes and insulating material.

FIG. 7F is a schematic of one embodiment of a sensor having twoelectrodes separated by a reference electrode or insulating material.

FIG. 7G is a schematic of another embodiment of a sensor having twoelectrodes separated by a reference electrode or insulating material.

FIG. 7H is a schematic of another embodiment of a sensor having twoelectrodes separated by a reference electrode or insulating material.

FIG. 7I is a schematic of another embodiment of a sensor having twoelectrodes separated by reference electrodes or insulating material.

FIG. 7J is a schematic of one embodiment of a sensor having twoelectrodes separated by a substantially X-shaped reference electrode orinsulating material.

FIG. 7K is a schematic of one embodiment of a sensor having twoelectrodes coated with insulating material, wherein one electrode has aspace for enzyme, the electrodes are separated by a distance D andcovered by a membrane system.

FIG. 7L is a schematic of one embodiment of a sensor having twoelectrodes embedded in an insulating material.

FIG. 7M is a schematic of one embodiment of a sensor having multipleworking electrodes and multiple reference electrodes.

FIG. 7N is a schematic of one step of the manufacture of one embodimentof a sensor having, embedded in insulating material, two workingelectrodes separated by a reference electrode, wherein the sensor istrimmed to a final size and/or shape.

FIG. 8A is a schematic on one embodiment of a sensor having two workingelectrodes coated with insulating material, and separated by a referenceelectrode.

FIG. 8B is a schematic of the second end (e.g., ex vivo terminus) of thesensor of FIG. 8A having a stepped connection to the sensor electronics.

FIG. 9A is a schematic of one embodiment of a sensor having two workingelectrodes and a substantially cylindrical reference electrode therearound, wherein the second end (the end connected to the sensorelectronics) of the sensor is stepped.

FIG. 9B is a schematic of one embodiment of a sensor having two workingelectrodes and an electrode coiled there around, wherein the second end(the end connected to the sensor electronics) of the sensor is stepped.

FIG. 10 is a schematic illustrating metabolism of glucose by GlucoseOxidase (GOx) and one embodiment of a diffusion barrier D thatsubstantially prevents the diffusion of H₂O₂ produced on a first side ofthe sensor (e.g., from a first electrode that has active GOx) to asecond side of the sensor (e.g., to the second electrode that lacksactive GOx).

FIG. 11 is a schematic illustrating one embodiment of a triple helicalcoaxial sensor having a stepped second terminus for engaging the sensorelectronics.

FIG. 12 is a graph that illustrates in vitro signal (raw counts)detected from a sensor having three bundled wire electrodes withstaggered working electrodes. Plus GOx (thick line)=the electrode withactive GOx. No GOx (thin line)=the electrode with inactive or no GOx.

FIG. 13 is a graph that illustrates in vitro signal (counts) detectedfrom a sensor having the configuration of the embodiment shown in FIG.7J (silver/silver chloride X-wire reference electrode separating twoplatinum wire working electrodes). Plus GOx (thick line)=the electrodewith active GOx. No GOx (thin line)=the electrode with inactive or noGOx.

FIG. 14 is a graph that illustrates an in vitro signal (counts) detectedfrom a dual-electrode sensor with a bundled configuration similar tothat shown in FIG. 7C (two platinum working electrodes and onesilver/silver chloride reference electrode, not twisted).

FIG. 15 is a graph that illustrates an in vivo signal (counts) detectedfrom a dual-electrode sensor with a bundled configuration similar tothat shown in FIG. 7C (two platinum working electrodes, not twisted, andone remotely disposed silver/silver chloride reference electrode).

FIG. 16 is a graph of interferant concentration versus time.

FIG. 17 is a schematic graph of current vs. voltage obtained from cyclicvoltammetry for hydrogen peroxide and acetaminophen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples illustrate some exemplaryembodiments of the disclosed invention in detail. Those of skill in theart will recognize that there are numerous variations and modificationsof this invention that are encompassed by its scope. Accordingly, thedescription of a certain exemplary embodiment should not be deemed tolimit the scope of the present invention.

Definitions

In order to facilitate an understanding of the preferred embodiments, anumber of terms are defined below.

The term “analyte” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to a substance or chemical constituent ina biological fluid (for example, blood, interstitial fluid, cerebralspinal fluid, lymph fluid or urine) that can be analyzed. Analytes mayinclude naturally occurring substances, artificial substances,metabolites, and/or reaction products. In some embodiments, the analytefor measurement by the sensor heads, devices, and methods disclosedherein is glucose. However, other analytes are contemplated as well,including but not limited to acarboxyprothrombin; acylcarnitine; adeninephosphoribosyl transferase; adenosine deaminase; albumin;alpha-fetoprotein; amino acid profiles (arginine (Krebs cycle),histidine/urocanic acid, homocysteine, phenylalanine/tyrosine,tryptophan); andrenostenedione; antipyrine; arabinitol enantiomers;arginase; benzoylecgonine (cocaine); biotimidase; biopterin; c-reactiveprotein; carnitine; carnosinase; CD4; ceruloplasmin; chenodeoxycholicacid; chloroquine; cholesterol; cholinesterase; conjugated 1-βhydroxy-cholic acid; cortisol; creatine kinase; creatine kinase MMisoenzyme; cyclosporin A; d-penicillamine; de-ethylchloroquine;dehydroepiandrosterone sulfate; DNA (acetylator polymorphism, alcoholdehydrogenase, alpha 1-antitrypsin, cystic fibrosis, Duchenne/Beckermuscular dystrophy, analyte-6-phosphate dehydrogenase,hemoglobinopathies, A,S,C,E, D-Punjab, beta-thalassemia, hepatitis Bvirus, HCMV, HIV-1, HTLV-1, Leber hereditary optic neuropathy, MCAD,RNA, PKU, Plasmodium vivax, sexual differentiation, 21-deoxycortisol);desbutylhalofantrine; dihydropteridine reductase; diptheria/tetanusantitoxin; erythrocyte arginase; erythrocyte protoporphyrin; esterase D;fatty acids/acylglycines; free β-human chorionic gonadotropin; freeerythrocyte porphyrin; free thyroxine (FT4); free tri-iodothyronine(FT3); fumarylacetoacetase; galactose/gal-1-phosphate;galactose-1-phosphate uridyltransferase; gentamicin; analyte-6-phosphatedehydrogenase; glutathione; glutathione perioxidase; glycocholic acid;glycosylated hemoglobin; halofantrine; hemoglobin variants;hexosaminidase A; human erythrocyte carbonic anhydrase I; 17alpha-hydroxyprogesterone; hypoxanthine phosphoribosyl transferase;immunoreactive trypsin; lactate; lead; lipoproteins ((a), B/A-1, β);lysozyme; mefloquine; netilmicin; phenobarbitone; phenyloin;phytanic/pristanic acid; progesterone; prolactin; prolidase; purinenucleoside phosphorylase; quinine; reverse tri-iodothyronine (rT3);selenium; serum pancreatic lipase; sissomicin; somatomedin C; specificantibodies (adenovirus, anti-nuclear antibody, anti-zeta antibody,arbovirus, Aujeszky's disease virus, dengue virus, Dracunculusmedinensis, Echinococcus granulosus, Entamoeba histolytica, enterovirus,Giardia duodenalisa, Helicobacter pylori, hepatitis B virus, herpesvirus, HIV-1, IgE (atopic disease), influenza virus, Leishmaniadonovani, leptospira, measles/mumps/rubella, Mycobacterium leprae,Mycoplasma pneumoniae, Myoglobin, Onchocerca volvulus, parainfluenzavirus, Plasmodium falciparum, poliovirus, Pseudomonas aeruginosa,respiratory syncytial virus, rickettsia (scrub typhus), Schistosomamansoni, Toxoplasma gondii, Trepenoma pallidium, Trypanosomacruzi/rangeli, vesicular stomatis virus, Wuchereria bancrofti, yellowfever virus); specific antigens (hepatitis B virus, HIV-1);succinylacetone; sulfadoxine; theophylline; thyrotropin (TSH); thyroxine(T4); thyroxine-binding globulin; trace elements; transferrin;UDP-galactose-4-epimerase; urea; uroporphyrinogen I synthase; vitamin A;white blood cells; and zinc protoporphyrin. Salts, sugar, protein, fat,vitamins, and hormones naturally occurring in blood or interstitialfluids may also constitute analytes in certain embodiments. The analytemay be naturally present in the biological fluid, for example, ametabolic product, a hormone, an antigen, an antibody, and the like.Alternatively, the analyte may be introduced into the body, for example,a contrast agent for imaging, a radioisotope, a chemical agent, afluorocarbon-based synthetic blood, or a drug or pharmaceuticalcomposition, including but not limited to insulin; ethanol; cannabis(marijuana, tetrahydrocannabinol, hashish); inhalants (nitrous oxide,amyl nitrite, butyl nitrite, chlorohydrocarbons, hydrocarbons); cocaine(crack cocaine); stimulants (amphetamines, methamphetamines, Ritalin,Cylert, Preludin, Didrex, PreState, Voranil, Sandrex, Plegine);depressants (barbituates, methaqualone, tranquilizers such as Valium,Librium, Miltown, Serax, Equanil, Tranxene); hallucinogens(phencyclidine, lysergic acid, mescaline, peyote, psilocybin); narcotics(heroin, codeine, morphine, opium, meperidine, Percocet, Percodan,Tussionex, Fentanyl, Darvon, Talwin, Lomotil); designer drugs (analogsof fentanyl, meperidine, amphetamines, methamphetamines, andphencyclidine, for example, Ecstasy); anabolic steroids; and nicotine.The metabolic products of drugs and pharmaceutical compositions are alsocontemplated analytes. Analytes such as neurochemicals and otherchemicals generated within the body may also be analyzed, such as, forexample, ascorbic acid, uric acid, dopamine, noradrenaline,3-methoxytyramine (3MT), 3,4-Dihydroxyphenylacetic acid (DOPAC),Homovanillic acid (HVA), 5-Hydroxytryptamine (5HT), and5-Hydroxyindoleacetic acid (FHIAA).

The term “continuous glucose sensor” as used herein is a broad term, andis to be given its ordinary and customary meaning to a person ofordinary skill in the art (and it is not to be limited to a special orcustomized meaning), and refers without limitation to a device thatcontinuously or continually measures glucose concentration, for example,at time intervals ranging from fractions of a second up to, for example,1, 2, or 5 minutes, or longer. It should be understood that continuousglucose sensors can continually measure glucose concentration withoutrequiring user initiation and/or interaction for each measurement, suchas described with reference to U.S. Pat. No. 6,001,067, for example.

The phrase “continuous glucose sensing” as used herein is a broad term,and is to be given its ordinary and customary meaning to a person ofordinary skill in the art (and it is not to be limited to a special orcustomized meaning), and refers without limitation to the period inwhich monitoring of plasma glucose concentration is continuously orcontinually performed, for example, at time intervals ranging fromfractions of a second up to, for example, 1, 2, or 5 minutes, or longer.

The term “biological sample” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a sample of a host body, forexample, blood, interstitial fluid, spinal fluid, saliva, urine, tears,sweat, tissue, and the like.

The term “host” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to plants or animals, for example humans.

The term “biointerface membrane” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a permeable or semi-permeablemembrane that can include one or more domains and is typicallyconstructed of materials of a few microns thickness or more, which canbe placed over the sensing region to keep host cells (for example,macrophages) from gaining proximity to, and thereby damaging themembrane system or forming a barrier cell layer and interfering with thetransport of glucose across the tissue-device interface.

The term “membrane system” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a permeable or semi-permeablemembrane that can be comprised of one or more domains and is typicallyconstructed of materials of a few microns thickness or more, which maybe permeable to oxygen and are optionally permeable to glucose. In oneexample, the membrane system comprises an immobilized glucose oxidaseenzyme, which enables an electrochemical reaction to occur to measure aconcentration of glucose.

The term “domain” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to regions of a membrane that can be layers,uniform or non-uniform gradients (for example, anisotropic), functionalaspects of a material, or provided as portions of the membrane.

The term “copolymer” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to polymers having two or more differentrepeat units and includes copolymers, terpolymers, tetrapolymers, andthe like.

The term “sensing region” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to the region of a monitoringdevice responsible for the detection of a particular analyte. In oneembodiment, the sensing region generally comprises a non-conductivebody, at least one electrode, a reference electrode and a optionally acounter electrode passing through and secured within the body forming anelectrochemically reactive surface at one location on the body and anelectronic connection at another location on the body, and a membranesystem affixed to the body and covering the electrochemically reactivesurface. In another embodiment, the sensing region generally comprises anon-conductive body, a working electrode (anode), a reference electrode(optionally can be remote from the sensing region), an insulatordisposed therebetween, and a multi-domain membrane affixed to the bodyand covering the electrochemically reactive surfaces of the working andoptionally reference electrodes.

The term “electrochemically reactive surface” as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and it is not to be limited to a specialor customized meaning), and refers without limitation to the surface ofan electrode where an electrochemical reaction takes place. In oneembodiment, a working electrode measures hydrogen peroxide creating ameasurable electronic current.

The term “electrochemical cell” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a device in which chemicalenergy is converted to electrical energy. Such a cell typically consistsof two or more electrodes held apart from each other and in contact withan electrolyte solution. Connection of the electrodes to a source ofdirect electric current renders one of them negatively charged and theother positively charged. Positive ions in the electrolyte migrate tothe negative electrode (cathode) and there combine with one or moreelectrons, losing part or all of their charge and becoming new ionshaving lower charge or neutral atoms or molecules; at the same time,negative ions migrate to the positive electrode (anode) and transfer oneor more electrons to it, also becoming new ions or neutral particles.The overall effect of the two processes is the transfer of electronsfrom the negative ions to the positive ions, a chemical reaction.

The term “electrode” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to a conductor through which electricityenters or leaves something such as a battery or a piece of electricalequipment. In one embodiment, the electrodes are the metallic portionsof a sensor (e.g., electrochemically reactive surfaces) that are exposedto the extracellular milieu, for detecting the analyte. In someembodiments, the term electrode includes the conductive wires or tracesthat electrically connect the electrochemically reactive surface toconnectors (for connecting the sensor to electronics) or to theelectronics.

The term “enzyme” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to a protein or protein-based molecule thatspeeds up a chemical reaction occurring in a living thing. Enzymes mayact as catalysts for a single reaction, converting a reactant (alsocalled an analyte herein) into a specific product. In one exemplaryembodiment of a glucose oxidase-based glucose sensor, an enzyme, glucoseoxidase (GOX) is provided to react with glucose (the analyte) and oxygento form hydrogen peroxide.

The term “co-analyte” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to a molecule required in an enzymaticreaction to react with the analyte and the enzyme to form the specificproduct being measured. In one exemplary embodiment of a glucose sensor,an enzyme, glucose oxidase (GOX) is provided to react with glucose andoxygen (the co-analyte) to form hydrogen peroxide.

The term “constant analyte” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to an analyte that remainsrelatively constant over a time period, for example over an hour to aday as compared to other variable analytes. For example, in a personwith diabetes, oxygen and urea may be relatively constant analytes inparticular tissue compartments relative to glucose, which is known tooscillate from about 40 to about 400 mg/dL during a 24-hour cycle.Although analytes such as oxygen and urea are known to oscillate to alesser degree, for example due to physiological processes in a host,they are substantially constant, relative to glucose, and can bedigitally filtered, for example low pass filtered, to minimize oreliminate any relatively low amplitude oscillations. Constant analytesother than oxygen and urea are also contemplated.

The term “proximal” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to near to a point of reference such as anorigin or a point of attachment. For example, in some embodiments of amembrane system that covers an electrochemically reactive surface, theelectrolyte domain is located more proximal to the electrochemicallyreactive surface than the resistance domain.

The term “distal” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to spaced relatively far from a point ofreference, such as an origin or a point of attachment. For example, insome embodiments of a membrane system that covers an electrochemicallyreactive surface, a resistance domain is located more distal to theelectrochemically reactive surfaces than the electrolyte domain.

The term “substantially” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a sufficient amount thatprovides a desired function. For example, the interference domain of thepreferred embodiments is configured to resist a sufficient amount ofinterfering species such that tracking of glucose levels can beachieved, which may include an amount greater than 50 percent, an amountgreater than 60 percent, an amount greater than 70 percent, an amountgreater than 80 percent, or an amount greater than 90 percent ofinterfering species.

The term “computer” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to machine that can be programmed tomanipulate data.

The term “modem” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to an electronic device for converting betweenserial data from a computer and an audio signal suitable fortransmission over a telecommunications connection to another modem.

The terms “processor module” and “microprocessor” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and they are not to be limited toa special or customized meaning), and refer without limitation to acomputer system, state machine, processor, or the like designed toperform arithmetic and logic operations using logic circuitry thatresponds to and processes the basic instructions that drive a computer.

The term “ROM” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to read-only memory, which is a type of datastorage device manufactured with fixed contents. ROM is broad enough toinclude EEPROM, for example, which is electrically erasable programmableread-only memory (ROM).

The term “RAM” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to a data storage device for which the orderof access to different locations does not affect the speed of access.RAM is broad enough to include SRAM, for example, which is static randomaccess memory that retains data bits in its memory as long as power isbeing supplied.

The term “A/D Converter” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to hardware and/or software thatconverts analog electrical signals into corresponding digital signals.

The term “RF transceiver” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a radio frequency transmitterand/or receiver for transmitting and/or receiving signals.

The terms “raw data stream” and “data stream” as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and they are not to be limited to aspecial or customized meaning), and refer without limitation to ananalog or digital signal directly related to the analyte concentrationmeasured by the analyte sensor. In one example, the raw data stream isdigital data in “counts” converted by an A/D converter from an analogsignal (for example, voltage or amps) representative of an analyteconcentration. The terms broadly encompass a plurality of time spaceddata points from a substantially continuous analyte sensor, whichcomprises individual measurements taken at time intervals ranging fromfractions of a second up to, for example, 1, 2, or 5 minutes or longer.In some embodiments, raw data includes one or more values (e.g., digitalvalue) representative of the current flow integrated over time (e.g.,integrated value), for example, using a charge counting device, or thelike.

The term “counts” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to a unit of measurement of a digital signal.In one example, a raw data stream measured in counts is directly relatedto a voltage (for example, converted by an A/D converter), which isdirectly related to current from a working electrode.

The term “electronic circuitry” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to the components (for example,hardware and/or software) of a device configured to process data. In thecase of an analyte sensor, the data includes biological informationobtained by a sensor regarding the concentration of the analyte in abiological fluid. U.S. Pat. Nos. 4,757,022, 5,497,772 and 4,787,398,which are hereby incorporated by reference in their entirety, describesuitable electronic circuits that can be utilized with devices ofcertain embodiments.

The term “potentiostat” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to an electrical system thatapplies a potential between the working and reference electrodes of atwo- or three-electrode cell at a preset value and measures the currentflow through the working electrode. Typically, the potentiostat forceswhatever current is necessary to flow between the working and referenceor counter electrodes to keep the desired potential, as long as theneeded cell voltage and current do not exceed the compliance limits ofthe potentiostat.

The terms “operably connected” and “operably linked” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and they are not to be limited toa special or customized meaning), and refer without limitation to one ormore components being linked to another component(s) in a manner thatallows transmission of signals between the components. For example, oneor more electrodes can be used to detect the amount of glucose in asample and convert that information into a signal; the signal can thenbe transmitted to an electronic circuit. In this case, the electrode is“operably linked” to the electronic circuit. These terms are broadenough to include wired and wireless connectivity.

The term “smoothing” and “filtering” as used herein are broad terms, andare to be given their ordinary and customary meaning to a person ofordinary skill in the art (and they are not to be limited to a specialor customized meaning), and refer without limitation to modification ofa set of data to make it smoother and more continuous and remove ordiminish outlying points, for example, by performing a moving average ofthe raw data stream.

The term “algorithm” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to the computational processes (forexample, programs) involved in transforming information from one stateto another, for example using computer processing.

The term “regression” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to finding a line in which a set of datahas a minimal measurement (for example, deviation) from that line.Regression can be linear, non-linear, first order, second order, and soforth. One example of regression is least squares regression.

The term “pulsed amperometric detection” as used herein is a broad term,and is to be given its ordinary and customary meaning to a person ofordinary skill in the art (and it is not to be limited to a special orcustomized meaning), and refers without limitation to an electrochemicalflow cell and a controller, which applies the potentials and monitorscurrent generated by the electrochemical reactions. The cell can includeone or multiple working electrodes at different applied potentials.Multiple electrodes can be arranged so that they face thechromatographic flow independently (parallel configuration), orsequentially (series configuration).

The term “calibration” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to the relationship and/or theprocess of determining the relationship between the sensor data andcorresponding reference data, which may be used to convert sensor datainto meaningful values substantially equivalent to the reference. Insome embodiments, namely in continuous analyte sensors, calibration maybe updated or recalibrated over time if changes in the relationshipbetween the sensor and reference data occur, for example due to changesin sensitivity, baseline, transport, metabolism, or the like.

The term “sensor analyte values” and “sensor data” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and they are not to be limited toa special or customized meaning), and refer without limitation to datareceived from a continuous analyte sensor, including one or moretime-spaced sensor data points.

The term “reference analyte values” and “reference data” as used hereinare broad terms, and are to be given their ordinary and customarymeaning to a person of ordinary skill in the art (and they are not to belimited to a special or customized meaning), and refer withoutlimitation to data from a reference analyte monitor, such as a bloodglucose meter, or the like, including one or more reference data points.In some embodiments, the reference glucose values are obtained from aself-monitored blood glucose (SMBG) test (for example, from a finger orforearm blood test) or an YSI (Yellow Springs Instruments) test, forexample.

The term “matched data pairs” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to reference data (for example,one or more reference analyte data points) matched with substantiallytime corresponding sensor data (for example, one or more sensor datapoints).

The terms “interferants” and “interfering species” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to effectsand/or species that interfere with the measurement of an analyte ofinterest in a sensor so as to produce a signal that does not accuratelyrepresent the analyte measurement. In one example of an electrochemicalsensor, interfering species are compounds with an oxidation potentialthat overlaps with that of the analyte to be measured, producing a falsepositive signal. In another example of an electrochemical sensor,interfering species are substantially non-constant compounds (e.g., theconcentration of an interfering species fluctuates over time).Interfering species include but are not limited to compounds withelectroactive acidic, amine or sulfhydryl groups, urea, lactic acid,phosphates, citrates, peroxides, amino acids, amino acid precursors orbreak-down products, nitric oxide (NO), NO-donors, NO-precursors,acetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyl dopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acidelectroactive species produced during cell metabolism and/or woundhealing, electroactive species that arise during body pH changes and thelike. Interferents that cause constant and/or non-constant noise areincluded in the definitions of “interferants” and “interfering species”.

The terms “substantial” and “substantially” as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and are not to be limited to aspecial or customized meaning), and refer without limitation to asufficient amount that provides a desired function. For example, theinterference domain of some embodiments is configured to substantiallyblock a sufficient amount of interfering species such that tracking ofglucose levels can be achieved, which may include an amount greater than50 percent, an amount greater than 60 percent, an amount greater than 70percent, an amount greater than 80 percent, and an amount greater than90 percent of interfering species.

The term “bifunctional” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to having or serving twofunctions. For example, in a needle-type analyte sensor, a metal wire isbifunctional because it provides structural support and acts as anelectrical conductor.

The term “function” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to an action or use for which something issuited or designed.

The term “electrical conductor” as used herein is a broad term, and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning) and refers without limitation to materials that contain movablecharges of electricity. When an electric potential difference isimpressed across separate points on a conductor, the mobile chargeswithin the conductor are forced to move, and an electric current betweenthose points appears in accordance with Ohm's law.

Accordingly, the term “electrical conductance” as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning) and refers without limitation to the propensity of amaterial to behave as an electrical conductor. In some embodiments, theterm refers to a sufficient amount of electrical conductance (e.g.material property) to provide a necessary function (electricalconduction).

The terms “insulative properties,” “electrical insulator” and“insulator” as used herein are broad terms, and are to be given theirordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning) and referswithout limitation to the tendency of materials that lack mobile chargesto prevent movement of electrical charges between two points. In oneexemplary embodiment, an electrically insulative material may be placedbetween two electrically conductive materials, to prevent movement ofelectricity between the two electrically conductive materials. In someembodiments, the terms refer to a sufficient amount of insulativeproperty (e.g., of a material) to provide a necessary function(electrical insulation). The terms “insulator” and “non-conductivematerial” can be used interchangeably herein.

The term “structural support” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning) and refers without limitation to the tendency of a material tokeep the sensor's structure stable or in place. For example, structuralsupport can include “weight bearing” as well as the tendency to hold theparts or components of a whole structure together. A variety ofmaterials can provide “structural support” to the sensor.

The term “diffusion barrier” as used herein is a broad term, and is tobe given its ordinary and customary meaning to a person of ordinaryskill in the art (and is not to be limited to a special or customizedmeaning) and refers without limitation to something that obstructs therandom movement of compounds, species, atoms, molecules, or ions fromone site in a medium to another. In some embodiments, a diffusionbarrier is structural, such as a wall that separates two workingelectrodes and substantially prevents diffusion of a species from oneelectrode to the other. In some embodiments, a diffusion barrier isspatial, such as separating working electrodes by a distancesufficiently large enough to substantially prevent a species at a firstelectrode from affecting a second electrode. In other embodiments, adiffusion barrier can be temporal, such as by turning the first andsecond working electrodes on and off, such that a reaction at a firstelectrode will not substantially affect the function of the secondelectrode.

The terms “integral,” “integrally,” “integrally formed,” integrallyincorporated,” “unitary” and “composite” as used herein are broad terms,and are to be given their ordinary and customary meaning to a person ofordinary skill in the art (and they are not to be limited to a specialor customized meaning), and refer without limitation to the condition ofbeing composed of essential parts or elements that together make awhole. The parts are essential for completeness of the whole. In oneexemplary embodiment, at least a portion (e.g., the in vivo portion) ofthe sensor is formed from at least one platinum wire at least partiallycovered with an insulative coating, which is at least partiallyhelically wound with at least one additional wire, the exposedelectroactive portions of which are covered by a membrane system (seedescription of FIG. 1B or 9B); in this exemplary embodiment, eachelement of the sensor is formed as an integral part of the sensor (e.g.,both functionally and structurally).

The term “coaxial” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to having a common axis, having coincidentaxes or mounted on concentric shafts.

The term “twisted” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and it is not to be limited to a special or customized meaning),and refers without limitation to united by having one part or end turnedin the opposite direction to the other, such as, but not limited to thetwisted strands of fiber in a string, yarn, or cable.

The term “helix” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to a spiral or coil, or something in the formof a spiral or coil (e.g. a corkscrew or a coiled spring). In oneexample, a helix is a mathematical curve that lies on a cylinder or coneand makes a constant angle with the straight lines lying in the cylinderor cone. A “double helix” is a pair of parallel helices intertwinedabout a common axis, such as but not limited to that in the structure ofDNA.

The term “in vivo portion” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a portion of a device that isto be implanted or inserted into the host. In one exemplary embodiment,an in vivo portion of a transcutaneous sensor is a portion of the sensorthat is inserted through the host's skin and resides within the host.

The terms “background,” “baseline,” and “noise” as used herein are broadterms, and are to be given their ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refer without limitation to acomponent of an analyte sensor signal that is not related to the analyteconcentration. In one example of a glucose sensor, the noise (e.g.,background) is composed substantially of signal contribution due tofactors other than glucose (for example, interfering species,non-reaction-related hydrogen peroxide, or other electroactive specieswith an oxidation potential that overlaps with hydrogen peroxide). Insome embodiments wherein a calibration is defined by solving for theequation y=mx+b, the value of b represents the baseline of the signal.In general, noise (e.g., background) comprises components related toconstant and non-constant factors (e.g., constant noise and non-constantnoise), including interfering species.

The term “constant noise” and “constant background” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and it is not to be limited to aspecial or customized meaning), and refer without limitation to thecomponent of the noise signal that remains relatively constant overtime. For example, certain electroactive compounds found in the humanbody are relatively constant factors (e.g., baseline of the host'sphysiology) and do not significantly adversely affect accuracy of thecalibration of the glucose concentration (e.g., they can be relativelyconstantly eliminated using the equation y=mx+b). In some circumstances,constant background noise can slowly drift over time (e.g., increases ordecreases), however this drift need not adversely affect the accuracy ofa sensor, for example, because a sensor can be calibrated andre-calibrated and/or the drift measured and compensated for.

The term “non-constant noise” or non-constant background” as used hereinare broad terms, and are to be given their ordinary and customarymeaning to a person of ordinary skill in the art (and it is not to belimited to a special or customized meaning), and refer withoutlimitation to a component of the background signal that is relativelynon-constant, for example, transient and/or intermittent. For example,certain electroactive compounds, are relatively non-constant (e.g.,intermittent interferents due to the host's ingestion, metabolism, woundhealing, and other mechanical, chemical and/or biochemical factors),which create intermittent (e.g., non-constant) “noise” on the sensorsignal that can be difficult to “calibrate out” using a standardcalibration equations (e.g., because the background of the signal doesnot remain constant).

The terms “inactive enzyme” or “inactivated enzyme” as used herein arebroad terms, and are to be given their ordinary and customary meaning toa person of ordinary skill in the art (and it is not to be limited to aspecial or customized meaning), and refer without limitation to anenzyme (e.g., glucose oxidase, GOx) that has been rendered inactive(e.g., “killed” or “dead”) and has no enzymatic activity. Enzymes can beinactivated using a variety of techniques known in the art, such as butnot limited to heating, freeze-thaw, denaturing in organic solvent,acids or bases, cross-linking, genetically changing enzymaticallycritical amino acids, and the like. In some embodiments, a solutioncontaining active enzyme can be applied to the sensor, and the appliedenzyme subsequently inactivated by heating or treatment with aninactivating solvent.

The term “non-enzymatic” as used herein is a broad term, and is to begiven their ordinary and customary meaning to a person of ordinary skillin the art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a lack of enzyme activity. Insome embodiments, a “non-enzymatic” membrane portion contains no enzyme;while in other embodiments, the “non-enzymatic” membrane portioncontains inactive enzyme. In some embodiments, an enzyme solutioncontaining inactive enzyme or no enzyme is applied.

The term “GOx” as used herein is a broad term, and is to be given theirordinary and customary meaning to a person of ordinary skill in the art(and it is not to be limited to a special or customized meaning), andrefers without limitation to the enzyme Glucose Oxidase (e.g., GOx is anabbreviation).

The term “mechanism” as used herein is a broad term, and is to be giventheir ordinary and customary meaning to a person of ordinary skill inthe art (and it is not to be limited to a special or customizedmeaning), and refers without limitation to a process, technique, orsystem for achieving a result. The term is not limited by the processes,techniques, or systems described herein, but also includes any process,technique, or system that can achieve a stated result.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that can varydepending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

Overview

Noise

Generally, implantable sensors measure a signal (e.g., counts) relatedto an analyte of interest in a host. For example, an electrochemicalsensor can measure glucose, creatinine, or urea in a host, such as ananimal, especially a human. Generally, the signal is convertedmathematically to a numeric value indicative of analyte status, such asanalyte concentration. However, it is not unusual for a sensor toexperience a certain level of noise. The term “noise” generally refersto a signal detected by the sensor that is substantially non-analyterelated (e.g., non-glucose related). In other words, things other thanthe analyte concentration substantially cause noise. Noise is clinicallyimportant because it can reduce sensor performance, such as by makingthe analyte concentration appear higher or lower than the actualconcentration. For example, if a host is hyperglycemic (e.g., bloodsugar too high, greater than ˜120 mg/dl) or euglycemic (e.g., ˜80-120mg/dl), noise can cause the host's blood sugar to appear higher than ittruly is, which can lead to improper treatment decisions, such as togive the host an excessive insulin dose. An excessive insulin dose, insome circumstances, can lead to a dangerous hypoglycemic state (e.g.,blood sugar too low, less than ˜80 mg/dl). In the case of a hypoglycemichost, noise can cause the hosts blood sugar to appear euglycemic or evenhyperglycemic, which can also lead to improper treatment decisions, suchas not eating when necessary or taking insulin, for example.Accordingly, since noise can cause error and reduce sensor performance,noise reduction is desirable.

Noise is comprised of two components, constant noise and non-constantnoise, and can be caused by a variety of factors, ranging frommechanical factors to biological factors. For example, it is known thatmacro- or micro-motion, ischemia, pH changes, temperature changes,pressure, stress, or even unknown mechanical, electrical, and/orbiochemical sources can cause noise. In general, “constant noise”(sometimes referred to as constant background or baseline) is caused byfactors that are relatively stable over time, including but not limitedto electroactive species that arise from generally constant (e.g.,daily) metabolic processes. In contrast, “non-constant noise” (sometimesreferred to as non-constant background) is caused by transient events,such as during wound healing or in response to an illness, or due toingestion (e.g., some drugs). In particular, noise can be caused by avariety of interfering species (constant or non-constant). Interferingspecies can be compounds, such as drugs that have been administered tothe host, or products of various host metabolic processes. Exemplaryinterferents include but are not limited to a variety of drugs (e.g.,acetaminophen), H₂O₂ from exterior sources, reactive metabolic species(e.g., reactive oxygen and nitrogen species, some hormones, etc.). Insome circumstances, constant noise-causing factors can have an affect onthe sensor signal similar to non-constant noise-causing factors, such aswhen the concentration of a constant noise-causing factor temporarilyincreases, such as due to temporary lack of lymph flow (see discussionof intermittent sedentary noise).

In some experiments of implantable glucose sensors, it was observed thatnoise increased when some hosts are intermittently sedentary, such asduring sleep or sitting for extended periods. When the host began movingagain, the noise quickly dissipated. Noise that occurs duringintermittent, sedentary periods (sometimes referred to as intermittentsedentary noise) can occur during relatively inactive periods, such assleeping. Non-constant, non-analyte-related factors can causeintermittent, sedentary noise, such as was observed in one exemplarystudy of non-diabetic individuals implanted with enzymatic-type glucosesensors built without enzyme. These sensors (without enzyme) could notreact with or measure glucose and therefore provided a signal due tonon-glucose effects (e.g., baseline, interferants, and/or noise). Duringsedentary periods (e.g., during sleep), extensive, sustained signal wasobserved on the sensors. Then, when the host got up and moved around,the signal rapidly corrected. Additional, in vitro experiments wereconducted to determine if a sensor (e.g., electrode) component mighthave leached into the area surrounding the sensor, but none wasdetected. From these results, it is believed that a host-producednon-analyte related reactant was diffusing to the electrodes andproducing the unexpected non-constant signal noise.

While not wishing to be bound by theory, it is believed that aconcentration increase of electroactive interferants, such aselectroactive metabolites from cellular metabolism and wound healing,can interfere with sensor function and cause noise observed during hostsedentary periods. For example, local lymph pooling, which can occurwhen a part of the body is compressed or when the body is inactive, cancause, in part, this local build up of interferants (e.g., electroactivemetabolites). Similarly, a local accumulation of wound healing metabolicproducts (e.g., at the site of sensor insertion) likely causes noise onthe sensor. Interferants can include but are not limited to compoundswith electroactive acidic, amine or sulfhydryl groups, urea, lacticacid, phosphates, citrates, peroxides, amino acids (e.g., L-arginine),amino acid precursors or break-down products, nitric oxide (NO),NO-donors, NO-precursors or other electroactive species or metabolitesproduced during cell metabolism and/or wound healing, for example. For amore complete discussion of noise and its sources, see co-pending U.S.patent application Ser. No. 11/503,367, filed Aug. 10, 2006 and entitled“ANALYTE SENSOR,” herein incorporated by reference in its entirety.

Noise can be difficult to remove from the sensor signal by calibrationusing standard calibration equations (e.g., because the background ofthe signal does not remain constant). Noise can significantly adverselyaffect the accuracy of the calibration of the analyte signal.Additionally noise, as described herein, can occur in the signal ofconventional sensors with electrode configurations that are notparticularly designed to measure noise substantially equally at bothactive and in-active electrodes (e.g., wherein the electrodes are spacedand/or non symmetrical, noise may not be equally measured and thereforenot easily removed using conventional dual electrode designs).

Noise can be recognized and/or analyzed in a variety of ways. Inpreferred embodiments, the sensor data stream is monitored, signalartifacts are detected and data processing is based at least in part onwhether or not a signal artifact has been detected, such as described inU.S. Patent Publication No. US-2005-0043598-A1. Additional discussioncan also be found in U.S. Patent Publication No. US-2007-0032706-A1,both herein incorporated by reference in their entirety.

Reduction of Noise

Noise can be recognized and substantially reduced and/or eliminated by avariety of sensor configurations and/or methods, such as by using 1)sensor configurations that block and/or remove the interferent, or thatspecifically detect the noise and 2) mathematical algorithms thatrecognize and/or remove the signal noise component. The preferredembodiments provide devices and methods for reducing and/or eliminatingnoise, such as by blocking interferent passage to the sensor'selectroactive surfaces, diluting and/or removing interferents around thesensor and mathematically determining and eliminating the noise signalcomponent. Those knowledgeable in the art will recognize that thevarious sensor structures (e.g., multiple working electrodes, membraneinterference domains, etc.), bioactive agents, algorithms and the likedisclosed herein can be employed in a plurality of combinations,depending upon the desired effect and the noise reduction strategyselected. In preferred embodiments, the sensor comprises at least twoworking electrodes (one with and one without enzyme over itselectroactive surface) and an interference domain configured tosubstantially block interferent passage therethrough, such that at leastsome interferent no longer has a substantial affect on sensormeasurements (e.g., at either working electrode). The term “interferencedomain,” as used herein is a broad term, and is to be given its ordinaryand customary meaning to a person of ordinary skill in the art (and itis not to be limited to a special or customized meaning), and referswithout limitation to any mechanism of the membrane system configured toreduce any kind of noise or interferants, such as constant and/ornon-constant noise. “Noise-reducing mechanisms” as used herein is abroad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and it is not to be limited to aspecial or customized meaning), and refers without limitation to anysensor system component configuration that reduces and/or eliminatesnoise on the sensor signal. Such structural configurations include butare not limited to electrode configurations (e.g., two or more workingelectrodes), membrane configurations (e.g., interference domain),algorithmic configurations (e.g., signal processing to remove anidentified noise component of the signal), and the like. In someembodiments, the interference domain is a component of the membranesystem, such as shown in FIGS. 2A and 2B. However, the interferencedomain can be disposed at any level (e.g., layer or domain) of themembrane system (e.g., more proximal or more distal to the electroactivesurfaces than as shown in FIGS. 2A and 2B). In some other embodiments,the interference domain is combined with an additional membrane domain,such as the resistance domain or the enzyme domain. A detaileddiscussion of the use of two or more electrodes to detect and reduceand/or eliminate noise can be found elsewhere herein, especially thesections entitled “Preferred Sensor Components” and “ExemplaryContinuous Sensor Configurations.” A detailed discussion ofnoise-blocking interference domains can be found in the section entitled“Interference Domain.”

Sensor Component Overview

The preferred embodiments provide a continuous analyte sensor thatmeasures a concentration of the analyte of interest or a substanceindicative of the concentration or presence of the analyte. In someembodiments, the analyte sensor is an invasive, minimally invasive, ornon-invasive device, for example a subcutaneous, transdermal, orintravascular device. In some embodiments, the analyte sensor mayanalyze a plurality of intermittent biological samples. The analytesensor may use any method of analyte-measurement, including enzymatic,chemical, physical, electrochemical, spectrophotometric, polarimetric,calorimetric, radiometric, or the like.

In general, analyte sensors provide at least one working electrode andat least one reference electrode, which are configured to measure asignal associated with a concentration of the analyte in the host, suchas described in more detail below, and as appreciated by one skilled inthe art. The output signal is typically a raw data stream that is usedto provide a useful value of the measured analyte concentration in ahost to the patient or doctor, for example. However, the analyte sensorsof the preferred embodiments comprise at least one additional workingelectrode configured to measure at least one additional signal, asdiscussed elsewhere herein. For example, in some embodiments, theadditional signal is associated with the baseline and/or sensitivity ofthe analyte sensor, thereby enabling monitoring of baseline and/orsensitivity changes that may occur in a continuous analyte sensor overtime.

In general, continuous analyte sensors define a relationship betweensensor-generated measurements (for example, current in nA or digitalcounts after A/D conversion) and a reference measurement (for example,mg/dL or mmol/L) that are meaningful to a user (for example, patient ordoctor). In the case of an implantable enzyme-based electrochemicalglucose sensor, the sensing mechanism generally depends on phenomenathat are linear with glucose concentration, for example: (1) diffusionof glucose through a membrane system (for example, biointerface membraneand membrane system) situated between implantation site and theelectrode surface, (2) an enzymatic reaction within the membrane system(for example, membrane system), and (3) diffusion of the H₂O₂ to thesensor. Because of this linearity, calibration of the sensor can beunderstood by solving an equation:y=mx+bwhere y represents the sensor signal (counts), x represents theestimated glucose concentration (mg/dL), m represents the sensorsensitivity to glucose (counts/mg/dL), and b represents the baselinesignal (counts). Because both sensitivity m and baseline (background) bchange over time in vivo calibration has conventionally required atleast two independent, matched data pairs (x₁, y₁; x₂, y₂) to solve form and b and thus allow glucose estimation when only the sensor signal, yis available. Matched data pairs can be created by matching referencedata (for example, one or more reference glucose data points from ablood glucose meter, or the like) with substantially time correspondingsensor data (for example, one or more glucose sensor data points) toprovide one or more matched data pairs, such as described in co-pendingU.S. Patent Publication No. US-2005-0027463-A1.

Accordingly, in some embodiments, the sensing region is configured tomeasure changes in sensitivity of the analyte sensor over time, whichcan be used to trigger calibration, update calibration, avoid inaccuratecalibration (for example, calibration during unstable periods), and/ortrigger filtering of the sensor data. Namely, the analyte sensor isconfigured to measure a signal associated with a non-analyte constant inthe host. Preferably, the non-analyte constant signal is measuredbeneath the membrane system on the sensor. In one example of a glucosesensor, a non-glucose constant that can be measured is oxygen, wherein ameasured change in oxygen transport is indicative of a change in thesensitivity of the glucose signal, which can be measured by switchingthe bias potential of the working electrode, an auxiliaryoxygen-measuring electrode, an oxygen sensor, or the like, as describedin more detail elsewhere herein.

Alternatively or additionally, in some embodiments, the sensing regionis configured to measure changes in the amount of background noise(e.g., baseline) in the signal, which can be used to triggercalibration, update calibration, avoid inaccurate calibration (forexample, calibration during unstable periods), and/or trigger filteringof the sensor data. In one example of a glucose sensor, the baseline iscomposed substantially of signal contribution due to factors other thanglucose (for example, interfering species, non-reaction-related hydrogenperoxide, or other electroactive species with an oxidation potentialthat overlaps with hydrogen peroxide). Namely, the glucose sensor isconfigured to measure a signal associated with the baseline (allnon-glucose related current generated) measured by sensor in the host.In some embodiments, an auxiliary electrode located beneath anon-enzymatic portion of the membrane system is used to measure thebaseline signal. In some embodiments, the baseline signal is subtractedfrom the glucose signal (which includes the baseline) to obtain thesignal contribution substantially only due to glucose. Subtraction maybe accomplished electronically in the sensor using a differentialamplifier, digitally in the receiver, and/or otherwise in the hardwareor software of the sensor or receiver as is appreciated by one skilledin the art, and as described in more detail elsewhere herein.

One skilled in the art appreciates that the above-described sensitivityand baseline signal measurements can be combined to benefit from bothmeasurements in a single analyte sensor.

Preferred Sensor Components

In general, sensors of the preferred embodiments describe a variety ofsensor configurations, wherein each sensor generally comprises two ormore working electrodes, a reference and/or counter electrode, aninsulator, and a membrane system configured to substantially reduceand/or eliminate noise and/or interferents. In general, the sensors canbe configured to continuously measure an analyte in a biological sample,for example, in subcutaneous tissue, in a host's blood flow, and thelike. Although a variety of exemplary embodiments are shown, one skilledin the art appreciates that the concepts and examples here can becombined, reduced, substituted, or otherwise modified in accordance withthe teachings of the preferred embodiments and/or the knowledge of oneskilled in the art.

Preferably, each exemplary sensor design (e.g., FIGS. 1A, 1B, 7A through9B, and 11) includes first and second working electrodes, wherein theworking electrodes are formed from known materials. In some embodiments,the sensor is configured with an architecture smaller than about 1 mm inat least one dimension. For example, in some embodiments, each electrodeis formed from a fine wire with a diameter of from about 0.001 or lessto about 0.010 inches or more, for example, and is formed from, e.g. aplated insulator, a plated wire, or bulk electrically conductivematerial. In preferred embodiments, the working electrodes comprisewires formed from a conductive material, such as platinum,platinum-iridium, palladium, graphite, gold, carbon, conductive polymer,alloys, or the like. Although the electrodes can by formed by a varietyof manufacturing techniques (bulk metal processing, deposition of metalonto a substrate, and the like), it can be advantageous to form theelectrodes from plated wire (e.g., platinum on steel wire) or bulk metal(e.g., platinum wire). It is believed that electrodes formed from bulkmetal wire provide superior performance (e.g., in contrast to depositedelectrodes), including increased stability of assay, simplifiedmanufacturability, resistance to contamination (e.g., which can beintroduced in deposition processes), and improved surface reaction(e.g., due to purity of material) without peeling or delamination.

Preferably, the first working electrode is configured to measure theconcentration of an analyte. In an enzymatic electrochemical sensor fordetecting glucose, for example, the first working electrode measures thehydrogen peroxide produced by an enzyme catalyzed reaction of theanalyte being detected and creates a measurable electronic current. Forexample, in the detection of glucose wherein glucose oxidase (GOX)produces hydrogen peroxide as a byproduct, hydrogen peroxide (H₂O₂)reacts with the surface of the working electrode producing two protons(2H⁺), two electrons (2e⁻) and one molecule of oxygen (O₂), whichproduces the electronic current being detected.

Preferably, each exemplary sensor design (e.g., FIGS. 1A, 1B, 7A through9B, and 11) includes at least one additional working electrodeconfigured to measure a non-analyte-related signal (e.g., baseline,background, etc.), to measure another analyte (e.g., oxygen), togenerate oxygen, and/or as a transport-measuring electrode, all of whichare described in more detail elsewhere herein. In general, theadditional working electrode(s) can be formed as described withreference to the first working electrode. In one embodiment, theauxiliary (additional) working electrode is configured to measure abackground signal, including constant and non-constant analyte signalcomponents.

Preferably, each exemplary sensor design (e.g., FIGS. 1A-1C, and 7Athrough 9B) includes a reference and/or counter electrode. In general,the reference electrode has a configuration similar to that describedelsewhere herein with reference to the first working electrode, howevermay be formed from materials, such as silver, silver/silver chloride,calomel, and the like. In some embodiments, the reference electrode isintegrally formed with the one or more working electrodes, however otherconfigurations are also possible (e.g. remotely located on the host'sskin, or otherwise in bodily fluid contact). In some exemplaryembodiments (e.g., FIGS. 1B and 9B, the reference electrode is helicallywound around other component(s) of the sensor system. In somealternative embodiments, the reference electrode is disposed remotelyfrom the sensor, such as but not limited to on the host's skin, asdescribed herein.

Preferably, each exemplary sensor design (e.g., FIGS. 1A-1C, 7A through9B, and 11) includes an insulator (e.g., non-conductive material) orsimilarly functional component. In some embodiments, one or moreelectrodes are covered with an insulating material, for example, anon-conductive polymer. Dip-coating, spray-coating, vapor-deposition, orother coating or deposition techniques can be used to deposit theinsulating material on the electrode(s). In some embodiments, theinsulator is a separate component of the system (e.g., see FIG. 7E) andcan be formed as is appreciated by one skilled in the art. In oneembodiment, the insulating material comprises parylene, which can be anadvantageous polymer coating for its strength, lubricity, and electricalinsulation properties. Generally, parylene is produced by vapordeposition and polymerization of para-xylylene (or its substitutedderivatives). In alternative embodiments, any suitable insulatingmaterial can be used, for example, fluorinated polymers,polyethyleneterephthalate, polyurethane, polyimide, other nonconductingpolymers, or the like. Glass or ceramic materials can also be employed.Other materials suitable for use include surface energy modified coatingsystems such as are marketed under the trade names AMC18, AMC148,AMC141, and AMC321 by Advanced Materials Components Express ofBellafonte, Pa.

Preferably, each exemplary sensor design (e.g., FIGS. 1A-1C, 7A through9B, and 11) includes exposed electroactive area(s). In embodimentswherein an insulator is disposed over one or more electrodes, a portionof the coated electrode(s) can be stripped or otherwise removed, forexample, by hand, excimer lasing, chemical etching, laser ablation,grit-blasting (e.g. with sodium bicarbonate or other suitable grit), andthe like, to expose the electroactive surfaces. Alternatively, a portionof the electrode can be masked prior to depositing the insulator inorder to maintain an exposed electroactive surface area. In oneexemplary embodiment, grit blasting is implemented to expose theelectroactive surfaces, preferably utilizing a grit material that issufficiently hard to ablate the polymer material, while beingsufficiently soft so as to minimize or avoid damage to the underlyingmetal electrode (e.g. a platinum electrode). Although a variety of“grit” materials can be used (e.g. sand, talc, walnut shell, groundplastic, sea salt, and the like), in some preferred embodiments, sodiumbicarbonate is an advantageous grit-material because it is sufficientlyhard to ablate, a coating (e.g., parylene) without damaging, anunderlying conductor (e.g., platinum). One additional advantage ofsodium bicarbonate blasting includes its polishing action on the metalas it strips the polymer layer, thereby eliminating a cleaning step thatmight otherwise be necessary. In some embodiments, the tip (e.g., end)of the sensor is cut to expose electroactive surface areas, without aneed for removing insulator material from sides of insulated electrodes.In general, a variety of surfaces and surface areas can be exposed.

Preferably, each exemplary sensor design (e.g., FIGS. 1A-1C, 7A through9B, and 11) includes a membrane system, such as those illustrated inFIGS. 2A and 2B. Preferably, a membrane system is deposited over atleast a portion of the electroactive surfaces of the sensor (workingelectrode(s) and optionally reference electrode) and provides protectionof the exposed electrode surface from the biological environment,diffusion resistance (limitation) of the analyte if needed, a catalystfor enabling an enzymatic reaction, limitation or blocking ofinterferents, and/or hydrophilicity at the electrochemically reactivesurfaces of the sensor interface. Some examples of suitable membranesystems are described in U.S. Patent Publication No. US-2005-0245799-A1.

In general, the membrane system includes a plurality of domains, forexample, one or more of an electrode domain 24, an interference domain26, an enzyme domain 28 (for example, including glucose oxidase), and aresistance domain 30, as shown in FIGS. 2A and 2B, and can include ahigh oxygen solubility domain, and/or a bioprotective domain (notshown), such as is described in more detail in U.S. Patent PublicationNo. US-2005-0245799-A1, and such as is described in more detail below.While the embodiment illustrated in FIGS. 2A and 2B shows theinterference domain between the electrode domain and the enzyme domain,the interference domain can be disposed more proximal or more distal tothe electroactive surfaces. For example, in some embodiments, theinterference domain is more distal to the electroactive surfaces thanthe enzyme domain. In some embodiments, the interference domain is themost distal layer/domain of the membrane system, relative to theelectroactive surfaces. In some embodiments, the interference domain canbe the most proximal domain/layer, relative to the electroactivesurfaces. In still other embodiments, the interference can be combinedwith one or more other membrane domains/layers. For example, in someembodiments, the interference domain and the resistance domain arecombined into a single domain that provides both interference blockingand control of analyte flux. In some embodiments, the membrane systemincludes one or more domains not illustrated in FIGS. 2A and 2B, such asbut not limited to a bioprotective domain (e.g., cell disruptive domain)and the like. On skilled in the art appreciates that a wide variety ofconfigurations and combinations encompassed by the preferredembodiments.

The membrane system can be deposited on the exposed electroactivesurfaces using known thin film techniques (for example, vapordeposition, spraying, electro-depositing, dipping, or the like). Inalternative embodiments, however, other vapor deposition processes(e.g., physical and/or chemical vapor deposition processes) can beuseful for providing one or more of the insulating and/or membranelayers, including ultrasonic vapor deposition, electrostatic deposition,evaporative deposition, deposition by sputtering, pulsed laserdeposition, high velocity oxygen fuel deposition, thermal evaporatordeposition, electron beam evaporator deposition, deposition by reactivesputtering molecular beam epitaxy, atmospheric pressure chemical vapordeposition (CVD), atomic layer CVD, hot wire CVD, low-pressure CVD,microwave plasma-assisted CVD, plasma-enhanced CVD, rapid thermal CVD,remote plasma-enhanced CVD, and ultra-high vacuum CVD, for example.However, the membrane system can be disposed over (or deposited on) theelectroactive surfaces using any known method, as will be appreciated byone skilled in the art.

In some embodiments, one or more domains of the membrane systems areformed from materials such as silicone, polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutyleneterephthalate (PBT), polymethylmethacrylate (PMMA), polyether etherketone (PEEK), polyurethanes, cellulosic polymers, polysulfones andblock copolymers thereof including, for example, di-block, tri-block,alternating, random and graft copolymers. U.S. Patent Publication No.US-2005-0245799-A1 describes biointerface and membrane systemconfigurations and materials that may be applied to the preferredembodiments.

Electrode Domain

In selected embodiments, the membrane system comprises an electrodedomain 24 (FIGS. 2A-2B). The electrode domain is provided to ensure thatan electrochemical reaction occurs between the electroactive surfaces ofthe working electrode and the reference electrode, and thus theelectrode domain is preferably situated more proximal to theelectroactive surfaces than the interference and/or enzyme domain.Preferably, the electrode domain includes a coating that maintains alayer of water at the electrochemically reactive surfaces of the sensor.In other words, the electrode domain is present to provide anenvironment between the surfaces of the working electrode and thereference electrode, which facilitates an electrochemical reactionbetween the electrodes. For example, a humectant in a binder materialcan be employed as an electrode domain; this allows for the fulltransport of ions in the aqueous environment. The electrode domain canalso assist in stabilizing the operation of the sensor by acceleratingelectrode start-up and drifting problems caused by inadequateelectrolyte. The material that forms the electrode domain can alsoprovide an environment that protects against pH-mediated damage that canresult from the formation of a large pH gradient due to theelectrochemical activity of the electrodes.

In one embodiment, the electrode domain includes a flexible,water-swellable, hydrogel film having a “dry film” thickness of fromabout 0.05 micron or less to about 20 microns or more, more preferablyfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably stillfrom about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. “Dryfilm” thickness refers to the thickness of a cured film cast from acoating formulation by standard coating techniques.

In certain embodiments, the electrode domain is formed of a curablemixture of a urethane polymer and a hydrophilic polymer. Particularlypreferred coatings are formed of a polyurethane polymer havingcarboxylate or hydroxyl functional groups and non-ionic hydrophilicpolyether segments, wherein the polyurethane polymer is crosslinked witha water-soluble carbodiimide (e.g.,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

In some preferred embodiments, the electrode domain is formed from ahydrophilic polymer such as polyvinylpyrrolidone (PVP). An electrodedomain formed from PVP has been shown to reduce break-in time of analytesensors; for example, a glucose sensor utilizing a cellulosic-basedinterference domain such as described in more detail below. Additionaldescription of using PVP to reduce break-in time can be found inco-pending U.S. patent application Ser. No. 11/654,140, filed Jan. 17,2007 and entitled “MEMBRANES FOR ANALYTE SENSOR” and U.S. PatentPublication No. US-2006-0229512-A1, which are incorporated herein byreference in their entirety.

Preferably, the electrode domain is deposited by vapor deposition, spraycoating, dip coating, or other thin film techniques on the electroactivesurfaces of the sensor. In one preferred embodiment, the electrodedomain is formed by dip-coating the electroactive surfaces in anelectrode layer solution and curing the domain for a time of from about15 minutes to about 30 minutes at a temperature of from about 40° C. toabout 55° C. (and can be accomplished under vacuum (e.g. 20 to 30mmHg)). In embodiments wherein dip-coating is used to deposit theelectrode domain, a preferred insertion rate of from about 1 to about 3inches per minute into the electrode layer solution, with a preferreddwell time of from about 0.5 to about 2 minutes in the electrode layersolution, and a preferred withdrawal rate of from about 0.25 to about 2inches per minute from the electrode layer solution provide a functionalcoating. However, values outside of those set forth above can beacceptable or even desirable in certain embodiments, for example,depending upon solution viscosity and solution surface tension, as isappreciated by one skilled in the art. In one embodiment, theelectroactive surfaces of the electrode system are dip-coated one time(one layer) and cured at 50° C. under vacuum for 20 minutes.

Although an independent electrode domain is described herein, in someembodiments sufficient hydrophilicity can be provided in theinterference domain and/or enzyme domain (the domain adjacent to theelectroactive surfaces) so as to provide for the full transport of ionsin the aqueous environment (e.g. without a distinct electrode domain).In these embodiments, an electrode domain is not necessary.

Interference Domain

As discussed elsewhere herein, noise can occur during the first fewhours or days after sensor implantation, such as during periods ofinactivity (e.g., intermittent, sedentary noise), and is believed to becaused by a local increase in interferants (e.g., electroactivemetabolites) that disrupts sensor function, resulting in apparentglucose signals that are generally unrelated to the host's glucoseconcentration. While not wishing to be bound by theory, it is believedthat the noise intensity and/or number of intermittent, sedentary noiseoccurrences can be reduced or eliminated by reducing the localconcentration of interferants, such as by incorporation of aninterference domain 26 into the membrane system 22. In general, the term“interference domain” includes any noise-reducing mechanism thatsubstantially blocks, reduces, eliminates, reacts with, or otherwisekeeps an interferant from reacting at the working electrode(s).Additionally, the noise-reducing mechanisms described herein, includingstructures, membrane materials, bioactive agents, and the like, whichcan reduce the effect of interfering species (noise) on the sensorsignal, can be considered at least a part of an “interference domain.”Some examples of interference domain structures are described herein inthis section entitled, “Interference Domain.” However, other knowninterference domain structures can be implemented with the dualelectrode sensor described herein. While the embodiments shown in FIGS.2A and 2B show the interference domain 26 located between the electrodeand enzyme domains, the interference domain can be disposed at any levelof the membrane system (e.g., more proximal or more distal to theelectroactive surfaces). For example, the interference domain can bedisposed between the enzyme domain and the resistance domain, betweenthe electroactive surfaces and the electrode domain, as the mostexterior membrane domain, etc. In some embodiments, any domain of themembrane system can be configured to function as an interference domainor combined with the interference domain. For example, the enzyme domainand interference domain can be combined into an enzyme-interferencedomain that performs the functions of an enzyme domain and aninterference domain. In one preferred embodiment, the sensor comprisestwo working electrodes (one with and one without enzyme) and a membranesystem comprising an interference domain configured to substantiallyreduce noise caused by one or more endogenous or exogenous interferents.

As illustrated in FIGS. 2A and 2B, the membrane system of the preferredembodiments includes an interference domain 26. In some preferredembodiments, an interference domain 26 is provided that substantiallyrestricts or blocks the flow of one or more interfering speciestherethrough. In some embodiments, the interference domain can beconfigured to reduce noise using, one, two or more noise-reducingmechanisms. For example, in some embodiments, the interference domain isconfigured to substantially block passage of at least one interferingspecies into the membrane system. In some embodiments, the interferencedomain is configured to substantially reduce the concentration of atleast one interferent, such as by increasing fluid bulk, forming a fluidpocket, or promoting increased bulk fluid flow. In some otherembodiments, the interference domain is configured to oxidize and/orreduce an interferent, such that the interferent no longer substantiallyaffects the sensor. In some embodiments, the interference domain isconfigured to reduce noise by combining two or more noise-reducingmechanisms, as described below. Some known interfering species for aglucose sensor, as described in more detail herein, includeacetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyldopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid. Insome embodiments, the interference domain of the preferred embodimentsis less permeable to one or more of the interfering species than to themeasured species, e.g. the product of an enzymatic reaction that ismeasured at the electroactive surface(s), such as but not limited toH₂O₂.

Cellulosic Polymer Materials

In one embodiment, the interference domain 26 is formed from one or morecellulosic derivatives. Cellulosic derivatives can include, but are notlimited to, cellulose esters and cellulose ethers. In general,cellulosic derivatives include polymers such as cellulose acetate,cellulose acetate butyrate, 2-hydroxyethyl cellulose, cellulose acetatephthalate, cellulose acetate propionate, cellulose acetate trimellitate,and the like, as well as their copolymers and terpolymers with othercellulosic or non-cellulosic monomers. Cellulose is a polysaccharidepolymer of β-D-glucose. While cellulosic derivatives are generallypreferred, other polymeric polysaccharides having similar properties tocellulosic derivatives can also be employed in the preferredembodiments.

In one preferred embodiment, the interference domain 26 is formed fromcellulose acetate butyrate. Cellulose acetate butyrate with a molecularweight of about 10,000 daltons to about 75,000 daltons, preferably fromabout 15,000, 20,000, or 25,000 daltons to about 50,000, 55,000, 60,000,65,000, or 70,000 daltons, and more preferably about 20,000 daltons isemployed. In certain embodiments, however, higher or lower molecularweights can be preferred. In some embodiments, a blend of two or morecellulose acetate butyrates having different molecular weights ispreferred. While a “blend” as defined herein (a composition of two ormore substances that are not substantially chemically combined with eachother and are capable of being separated) is generally preferred, incertain embodiments a single polymer incorporating differentconstituents (e.g., separate constituents as monomeric units and/orsubstituents on a single polymer chain) can be employed instead.Additionally, a casting solution or dispersion of cellulose acetatebutyrate at a wt. % of from about 5% to about 25%, preferably from about5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15% to about 16%, 17%,18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%, and more preferably from about5% to about 15% is preferred. Preferably, the casting solution includesa solvent or solvent system, for example an acetone:ethanol solventsystem. Higher or lower concentrations can be preferred in certainembodiments. In alternative embodiments, a single solvent (e.g. acetone)is used to form a symmetrical membrane domain. A single solvent is usedin casting solutions for forming symmetric membrane layer(s). Aplurality of layers of cellulose acetate butyrate can be advantageouslycombined to form the interference domain in some embodiments, forexample, three layers can be employed. It can be desirable to employ amixture of cellulose acetate butyrate components with differentmolecular weights in a single solution, or to deposit multiple layers ofcellulose acetate butyrate from different solutions comprising celluloseacetate butyrate of different molecular weights, differentconcentrations, and/or different chemistries (e.g. functional groups).It can also be desirable to include additional substances in the castingsolutions or dispersions, e.g. functionalizing agents, crosslinkingagents, other polymeric substances, substances capable of modifying thehydrophilicity/hydrophobicity of the resulting layer, and the like.

In one alternative embodiment, the interference domain 26 is formed fromcellulose acetate. Cellulose acetate with a molecular weight of about30,000 daltons or less to about 100,000 daltons or more, preferably fromabout 35,000, 40,000, or 45,000 daltons to about 55,000, 60,000, 65,000,70,000, 75,000, 80,000, 85,000, 90,000, or 95,000 daltons, and morepreferably about 50,000 daltons is preferred. In some embodiments, ablend of two or more cellulose acetates having different molecularweights is preferred. Additionally, a casting solution or dispersion ofcellulose acetate at a weight percent of about 3% to about 10%,preferably from about 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, or 6.5% toabout 7.5%, 8.0%, 8.5%, 9.0%, or 9.5%, and more preferably about 8% ispreferred. In certain embodiments, however, higher or lower molecularweights and/or cellulose acetate weight percentages can be preferred. Itcan be desirable to employ a mixture of cellulose acetates withmolecular weights in a single solution, or to deposit multiple layers ofcellulose acetate from different solutions comprising cellulose acetatesof different molecular weights, different concentrations, or differentchemistries (e.g. functional groups). It can also be desirable toinclude additional substances in the casting solutions or dispersionssuch as described in more detail above.

In addition to forming an interference domain from only celluloseacetate(s) or only cellulose acetate butyrate(s), the interferencedomain 26 can be formed from combinations or blends of cellulosicderivatives, such as but not limited to cellulose acetate and celluloseacetate butyrate, or combinations of layer(s) of cellulose acetate andlayer(s) of cellulose acetate butyrate. In some embodiments, a blend ofcellulosic derivatives (for formation of an interference domain)includes up to about 10 wt. % or more of cellulose acetate. For example,about 1, 2, 3, 4, 5, 6, 7, 8, 9 wt. % or more cellulose acetate ispreferred, in some embodiments. In some embodiments, the cellulosicderivatives blend includes from about 90 wt. % or less to about 100 wt.% cellulose acetate butyrate. For example, in some embodiments, theblend includes about 91, 92, 93, 94, 95, 96, 97, 98 or 99 wt. %cellulose acetate butyrate. In some embodiments, the cellulosicderivative blend includes from about 1.5, 2.0, 2.5, 3.0 or 3.5 wt. %cellulose acetate to about 98.5, 98.0, 97.5, 97.0 or 96.5 wt. %cellulose acetate butyrate. In other embodiments, the blend includesfrom about 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5 or 8 wt. % cellulose acetateto about 96, 95.5, 95, 94.5, 94, 93.3, 93, 92.5 or 92 wt. % celluloseacetate butyrate. In still other embodiments, the blend includes fromabout 8.5, 9.0, 9.5, 10.0, 10.5 or 11.0 wt. % cellulose acetate to about91.5, 91.0, 90.5, 90, 89.5 or 89 wt. % cellulose acetate butyrate.

In some embodiments, preferred blends of cellulose acetate and celluloseacetate butyrate contain from about 1.5 parts or less to about 60 partsor more cellulose acetate butyrate to one part of cellulose acetate. Insome embodiments, a blend contains from about 2 parts to about 40 partscellulose acetate butyrate to one part cellulose acetate. In otherembodiments, about 4, 6, 8, 10, 12, 14, 16, 18 or 20 parts celluloseacetate butyrate to one part cellulose acetate is preferred forformation of the interference domain 26. In still other embodiments, ablend having from 22, 24, 26, 28, 30, 32, 34, 36 or 38 parts celluloseacetate butyrate to one part cellulose acetate is preferred. As isdiscussed elsewhere herein, cellulose acetate butyrate is relativelymore hydrophobic than cellulose acetate. Accordingly, the celluloseacetate/cellulose acetate butyrate blend contains substantially morehydrophobic than hydrophilic components.

Cellulose acetate butyrate is a cellulosic polymer having both acetyland butyl groups, in addition to hydroxyl groups. Acetyl groups are morehydrophilic than butyl groups, and hydroxyl groups are more hydrophilicthan both acetyl and butyl groups. Accordingly, the relative amounts ofacetyl, butyl and hydroxyl groups can be used to modulate thehydrophilicity/hydrophobicity of the cellulose acetate butyrate of thecellulose acetate/cellulose acetate butyrate blend. A cellulose acetatebutyrate can be selected based on the compound's relative amounts ofacetate, butyrate and hydroxyl groups; and a cellulose acetate can beselected based on the compounds relative amounts of acetate and hydroxylgroups. For example, in some embodiments, a cellulose acetate butyratehaving about 35% or less acetyl groups, about 10% to about 25% butylgroups, and hydroxyl groups making up the remainder is preferred forformation of the interference domain 26. In other embodiments acellulose acetate butyrate having from about 25% to about 34% acetylgroups and from about 15 to about 20% butyl groups is preferred. Instill other embodiments, the preferred cellulose acetate butyratecontains from about 28% to about 30% acetyl groups and from about 16 toabout 18% butyl groups. In yet another embodiment, the cellulose acetatebutyrate can have no acetate groups and from about 20% to about 60%butyrate groups. In yet another embodiment, the cellulose acetatebutyrate has about 55% butyrate groups and no acetate groups.

While an asymmetric interference domain can be used in some alternativeembodiments, a symmetrical interference domain 26 (e.g. ofcellulosic-derivative blends, such as but not limited to blends ofcellulose acetate components and cellulose acetate butyrate components)is preferred in some embodiments. Symmetrical membranes are uniformthroughout their entire structure, without gradients of pore densitiesor sizes, or a skin on one side but not the other, for example. Invarious embodiments, a symmetrical interference domain 26 can be formedby the appropriate selection of a solvent (e.g. no anti-solvent isused), for making the casting solution. Appropriate solvents includesolvents belonging to the ketone family that are able to solvate thecellulose acetate and cellulose acetate butyrate. The solvents includebut are not limited to acetone, methyl ethyl ketone, methyl n-propylketone, cyclohexanone, and diacetone alcohol. Other solvents, such asfurans (e.g. tetra-hydro-furan and 1,4-dioxane), may be preferred insome embodiments. In one exemplary embodiment, between about 7 wt. % andabout 9 wt. % solids (e.g., a blend of cellulosic derivatives, such ascellulose acetate and cellulose acetate butyrate) are blended with asingle solvent (e.g., acetone), to form the casting solution for asymmetrical interference domain. In another embodiment, from about 10 toabout 15% solids are blended with acetone to form the casting solution.In yet another embodiment, from about 16 to about 18% solids are blendedwith acetone to form the casting solution. A relatively lower or greaterweight percent of solids is preferred to form the casting solution, insome embodiments.

The casting solution can be applied either directly to the electroactivesurface(s) of the sensor or on top of an electrode domain layer (ifincluded in the membrane system). The casting solution can be appliedusing any known thin film technique, as discussed elsewhere herein.Additionally, in various embodiments, a symmetrical interference domain26 includes at least one layer; and in some embodiments, two, three ormore layers are formed by the sequential application and curing of thecasting solution.

The concentration of solids in the casting solution can be adjusted todeposit a sufficient amount of solids on the electrode in one layer(e.g. in one dip or spray) to form a membrane layer with sufficientblocking ability, such that the equivalent glucose signal of aninterferent (e.g. compounds with an oxidation or reduction potentialthat overlaps with that of the measured species (e.g. H₂O₂)), measuredby the sensor, is about 60 mg/dL or less. For example, in someembodiments, the casting solution's percentage of solids is adjustedsuch that only a single layer (e.g. dip one time) is required to deposita sufficient amount of the cellulose acetate/cellulose acetate butyrateblend to form a functional symmetric interference domain thatsubstantially blocks passage therethrough of at least one interferent,such as but not limited to acetaminophen, ascorbic acid, dopamine,ibuprofen, salicylic acid, tolbutamide, tetracycline, creatinine, uricacid, ephedrine, L-dopa, methyl dopa and tolazamide. In someembodiments, the amount of interference domain material deposited by assingle dip is sufficient to reduce the equivalent glucose signal of theinterferant (e.g. measured by the sensor) to about 60 mg/dl or less. Inpreferred embodiments, the interferent's equivalent glucose signalresponse (measured by the sensor) is 50 mg/dl or less. In more preferredembodiments, the interferent produces an equivalent glucose signalresponse of 40 mg/dl or less. In still more preferred embodiments, theinterferent produces an equivalent glucose signal response of less thanabout 30, 20 or 10 mg/dl. In one exemplary embodiment, the interferencedomain is configured to substantially block acetaminophen passagetherethrough, wherein the equivalent glucose signal response of theacetaminophen is less than about 30 mg/dl.

In alternative embodiments, the interference domain is configured tosubstantially block a therapeutic dose of acetaminophen. The term“therapeutic dose” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning), andrefers without limitation to the quantity of any substance required toeffect the cure of a disease, to relieve pain, or that will correct themanifestations of a deficiency of a particular factor in the diet, suchas the effective dose used with therapeutically applied compounds, suchas drugs. For example, a therapeutic dose of acetaminophen can be anamount of acetaminophen required to relieve headache pain or reduce afever. As a further example, 1,000 mg of acetaminophen taken orally,such as by swallowing two 500 mg tablets of acetaminophen, is thetherapeutic dose frequently taken for headaches. In some embodiments,the interference membrane is configured to block a therapeutic dose ofacetaminophen, wherein the equivalent glucose signal response of theacetaminophen is less than about 60 mg/dl. In a preferred embodiment,the interference membrane is configured to block a therapeutic dose ofacetaminophen, wherein the equivalent glucose signal response of theacetaminophen is less than about 40 mg/dl. In a more preferredembodiment, the interference membrane is configured to block atherapeutic dose of acetaminophen, wherein the equivalent glucose signalresponse of the acetaminophen is less than about 30 mg/dl.

While not wishing to be bound by theory, it is believed that, withrespect to symmetrical cellulosic-based membranes, there is an inverselyproportional balance between interferent blocking and analytesensitivity. Namely, changes to the interference domain configurationthat increase interferent blocking can result in a correspondingdecrease in sensor sensitivity. Sensor sensitivity is discussed in moredetail elsewhere herein. It is believed that the balance betweeninterferent blocking and sensor sensitivity is dependent upon therelative proportions of hydrophobic and hydrophilic components of themembrane layer (e.g. the interference domain), with sensors having morehydrophobic interference domains having increased interferent blockingbut reduces sensitivity; and sensors having more hydrophilicinterference domains having reduced interferent blocking but increasedsensitivity. It is believed that the hydrophobic and hydrophiliccomponents of the interference domain can be balanced, to promote adesired level of interferent blocking while at the same time maintaininga desired level of analyte sensitivity. The interference domainhydrophobe-hydrophile balance can be manipulated and/or maintained bythe proper selection and blending of the hydrophilic and hydrophobicinterference domain components (e.g. cellulosic derivatives havingacetyl, butyryl, propionyl, methoxy, ethoxy, propoxy, hydroxyl,carboxymethyl, and/or carboxyethyl groups). For example, celluloseacetate is relatively more hydrophilic than cellulose acetate butyrate.In some embodiments, increasing the percentage of cellulose acetate (orreducing the percentage of cellulose acetate butyrate) can increase thehydrophilicity of the cellulose acetate/cellulose acetate butyrateblend, which promotes increased permeability to hydrophilic species,such as but not limited to glucose, H₂O₂ and some interferents (e.g.acetaminophen). In another embodiment, the percentage of celluloseacetate butyrate is increased to increase blocking of interferants, butless permeability to some desired molecules, such as H₂O₂ and glucose,is also reduced.

One method, of manipulating the hydrophobe-hydrophile balance of theinterference domain, is to select the appropriate percentages of acetylgroups (relatively more hydrophilic than butyl groups), butyl groups(relatively more hydrophobic than acetyl groups) and hydroxyl groups ofthe cellulose acetate butyrate used to form the interference domain 26.For example, increasing the percentage of acetate groups on thecellulose acetate butyrate will make the cellulose acetate butyrate morehydrophilic. In another example, increasing the percentage of butylgroups on the cellulose acetate butyrate will make the cellulose acetatebutyrate more hydrophobic. In yet another example, increasing thepercentage of hydroxyl groups will increase the hydrophilicity of thecellulose acetate butyrate. Accordingly, the selection of a celluloseacetate butyrate that is more or less hydrophilic (or more or lesshydrophobic) can modulate the over-all hydrophilicity of the celluloseacetate/cellulose acetate butyrate blend. In one exemplary embodiment,an interference domain can be configured to be relatively morehydrophobic (and therefore block interferants more strongly) by reducingthe percentage of acetyl or hydroxyl groups or by increasing thepercentage of butyl groups on the cellulose acetate butyrate used in thecasting solution (while maintaining the relative ratio of celluloseacetate to cellulose acetate butyrate).

In some alternative embodiments, the interference domain is formed of ablend of cellulosic derivatives, wherein the hydrophilic and hydrophobiccomponents of the interference domain are balanced, such that theglucose sensitivity is from about 1 pA/mg/dL to about 100 pA/mg/dL, andat least one interferent is sufficiently blocked from passage throughthe interference domain such that the equivalent glucose signal responseof the at least one interferent is less than about 60 mg/dL. In apreferred embodiment, the glucose sensitivity is from about 5 pA/mg/dLto about 25 pA/mg/dL. In a more preferred embodiments, the glucosesensitivity is from about 5 pA/mg/dL to about 25 pA/mg/dL and theequivalent glucose signal response of the at least one interferent isless than about 40 mg/dL. In a still more preferred embodiments, theglucose sensitivity is from about 5 pA/mg/dL to about 25 pA/mg/dL andthe equivalent glucose signal response of the at least one interferentis less than about 30 mg/dL. In some embodiments, the balance betweenhydrophilic and hydrophobic components of the interference domain can beachieved by adjusting the amounts of hydrophilic and hydrophobiccomponents, relative to each other, as well as adjusting the hydrophilicand hydrophobic groups (e.g., acetyl, butyryl, propionyl, methoxy,ethoxy, propoxy, hydroxyl, carboxymethyl, and/or carboxyethyl groups) ofthe components themselves (e.g., cellulosic derivatives, such as but notlimited to cellulose acetate and cellulose acetate butyrate).

In some alternative embodiments, additional polymers, such as Nafion®,can be used in combination with cellulosic derivatives to provideequivalent and/or enhanced function of the interference domain 26. Asone example, a layer of a 5 wt. % Nafion® casting solution was appliedover a previously applied (e.g., and cured) layer of 8 wt. % celluloseacetate, e.g. by dip coating at least one layer of cellulose acetate andsubsequently dip coating at least one layer Nafion® onto a needle-typesensor such as described with reference to the preferred embodiments.Any number of coatings or layers formed in any order may be suitable forforming the interference domain of the preferred embodiments.

In some alternative embodiments, more than one cellulosic derivative canbe used to form the interference domain 26 of the preferred embodiments.In general, the formation of the interference domain on a surfaceutilizes a solvent or solvent system, in order to solvate the cellulosicderivative(s) (or other polymer) prior to film formation thereon. Inpreferred embodiments, acetone and ethanol are used as solvents forcellulose acetate; however one skilled in the art appreciates thenumerous solvents that are suitable for use with cellulosic derivatives(and other polymers). Additionally, one skilled in the art appreciatesthat the preferred relative amounts of solvent can be dependent upon thecellulosic derivative (or other polymer) used, its molecular weight, itsmethod of deposition, its desired thickness, and the like. However, apercent solute of from about 1 wt. % to about 25 wt. % is preferablyused to form the interference domain solution so as to yield aninterference domain having the desired properties. The cellulosicderivative (or other polymer) used, its molecular weight, method ofdeposition, and desired thickness can be adjusted, depending upon one ormore other of the parameters, and can be varied accordingly as isappreciated by one skilled in the art.

In some alternative embodiments, other polymer types that can beutilized for the interference domain 26 including polyurethanes,polymers having pendant ionic groups, and polymers having controlledpore size, for example. In one such alternative embodiment, theinterference domain includes a thin, hydrophobic membrane that isnon-swellable and restricts diffusion of high molecular weight species.The interference domain 26 is permeable to relatively low molecularweight substances, such as hydrogen peroxide, but restricts the passageof higher molecular weight substances, including glucose and ascorbicacid. Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system of the preferredembodiments are described in U.S. Pat. No. 7,074,307, U.S. PatentPublication No. 2005-0176136-A1, U.S. Pat. No. 7,081,195, and U.S.Patent Publication No. 2005-0143635-A1. In some alternative embodiments,a distinct interference domain is not included.

In some embodiments, the interference domain 26 is deposited eitherdirectly onto the electroactive surfaces of the sensor or onto thedistal surface of the electrode domain, for a domain thickness of fromabout 0.05 micron or less to about 20 microns or more, more preferablyfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably stillfrom about 1, 1.5 or 2 microns to about 2.5 or 3 microns. Thickermembranes can also be desirable in certain embodiments, but thinnermembranes are generally preferred because they have a lower impact onthe rate of diffusion of hydrogen peroxide from the enzyme membrane tothe electrodes. In some embodiments, the interference domain can bedeposited either more proximal or more distal than the electrode domain,relative to the electroactive surfaces, depending upon the interferencedomain composition and membrane system configuration.

In general, the membrane systems of the preferred embodiments can beformed and/or deposited on the exposed electroactive surfaces (e.g. oneor more of the working and reference electrodes) using known thin filmtechniques (for example, casting, spray coating, drawing down,electro-depositing, dip coating, and the like), however casting or otherknown application techniques can also be utilized. Preferably, theinterference domain is deposited by spray or dip coating. In oneexemplary embodiment of a needle-type (transcutaneous) sensor such asdescribed herein, the interference domain is formed by dip coating thesensor into an interference domain solution using an insertion rate offrom about 0.5 inch/min to about 60 inches/min, preferably 1 inch/min, adwell time of from about 0 minute to about 2 minutes, preferably about 1minute, and a withdrawal rate of from about 0.5 inch/minute to about 60inches/minute, preferably about 1 inch/minute, and curing (drying) thedomain from about 1 minute to about 30 minutes, preferably from about 3minutes to about 15 minutes (and can be accomplished at room temperatureor under vacuum (e.g., 20 to 30 mmHg)). In one exemplary embodimentincluding cellulose acetate butyrate interference domain, a 3-minutecure (i.e., dry) time is preferred between each layer applied. Inanother exemplary embodiment employing a cellulose acetate interferencedomain, a 15-minute cure (i.e., dry) time is preferred between eachlayer applied.

In some embodiments, the dip process can be repeated at least one timeand up to 10 times or more. In other embodiments, only one dip ispreferred. The preferred number of repeated dip processes depends uponthe cellulosic derivative(s) used, their concentration, conditionsduring deposition (e.g., dipping) and the desired thickness (e.g.,sufficient thickness to provide functional blocking of certaininterferents), and the like. In some embodiments, 1 to 3 microns may bepreferred for the interference domain thickness, however, values outsideof these can be acceptable or even desirable in certain embodiments, forexample, depending upon viscosity and surface tension, as is appreciatedby one skilled in the art. In one exemplary embodiment, an interferencedomain is formed from three layers of cellulose acetate butyrate. Inanother exemplary embodiment, an interference domain is formed from 10layers of cellulose acetate. In another embodiment, an interferencedomain is formed from 1 layer of a blend of cellulose acetate andcellulose acetate butyrate. In alternative embodiments, the interferencedomain can be formed using any known method and combination of celluloseacetate and cellulose acetate butyrate, as will be appreciated by oneskilled in the art.

In some embodiments, the electroactive surface can be cleaned prior toapplication of the interference domain 26. In some embodiments, theinterference domain 26 of the preferred embodiments can be useful as abioprotective or biocompatible domain, namely, a domain that interfaceswith host tissue when implanted in an animal (e.g. a human) due to itsstability and biocompatibility.

Silicone/Hydrophilic Polymer Blend Materials

It is believed that incorporation of a silicone-hydrophilic polymerblend into the membrane system can substantially reduce and/or eliminatenoise, such as by substantially blocking and/or slowing (e.g., reducingthe diffusion rate) the passage of an interferent therethrough. Inpreferred embodiments, a sensor having two or more working electrodes,such as but not limited to those illustrated in FIGS. 1B, 7A1-9B and 11,includes a membrane system 22 wherein the interference domain comprisesa blend of a silicone polymer with a hydrophilic polymer configured toreduce noise-causing species, such as constant and/or non-constantnoise-causing species. In some embodiments, additional membranedomains/layers comprise a blend of a silicone polymer with a hydrophilicpolymer configured to reduce noise-causing species. In some preferredembodiments, the sensor includes a silicone-hydrophilic polymer blendmembrane domain and/or layer (e.g., an interference domain) comprising amicellar jacket structure (described elsewhere herein). While notwishing to be bound by theory, it is believed that membrane domainscomprising a silicone-hydrophilic polymer blend can reduce noise byblocking and/or suppressing passage of at least one interfering speciesinto the membrane system, while at the same time allowing for and/orpromoting the transport of the analyte (e.g., glucose or other suchwater-soluble molecules, such as drugs).

By “hydrophilic polymer,” it is meant that the polymer has an affinityfor water, due to the presence of one or more hydrophilic substituents,and generally is primarily soluble in water or has a tendency to absorbwater. In one example, the hydrophilic component of a hydrophilicpolymer promotes the movement of water and/or compounds in the water(e.g., by diffusion or other means) through a membrane formed of thehydrophilic polymer, such as by lowering the thermodynamic barrier tomovement of compounds in the water into the membrane.

In some embodiments, hydrophilic polymers includehydrophilic-hydrophobic polymers. Generally, the terms“hydrophilic-hydrophobic” and “hydrophobic-hydrophilic” are usedinterchangeably herein (are not meant to imply that either thehydrophilic or the hydrophobic substituents are the major component ofthe polymer) and refer to the property of having both hydrophilic andhydrophobic substituents and/or characteristics in a single molecule,such as, for example, a polymer.

The hydrophilic and hydrophobic substituents of a polymer can affect thepolymer's behavior in certain circumstances, such as but not limited tosilicone/hydrophilic-hydrophobic blend materials and micellar jackets,which are discussed elsewhere herein. Using PEO-PPO-PEO as an exemplarypolymer, the polymer's major component (PEO) is hydrophilic and canprovide an overall hydrophilic character to the molecule (e.g., themolecule generally behaves in a hydrophilic manner). However, thehydrophobic component (PPO) of the polymer makes it possible for thepolymer to have some hydrophobic character (e.g., for portions of themolecule to behave in the manner of a hydrophobic molecule), in somesituations. In some circumstances, such as formation of micellar jacketsin a silicone/hydrophilic-hydrophobic blend material, the polymerself-organizes, relative to the silicone (e.g., silicone globule(s))such that the hydrophobic PPO is adjacent to the silicone (which ishydrophobic) and the two PEO groups project away from the silicone(e.g., due to thermodynamic forces). Depending upon the circumstance(e.g., the polymer selected), variations of the micellar jacketstructure described above (e.g., opposite orientations) are possible.For example, it is believed that in a mixture of PPO-PEO-PPO andsilicone, the PPO groups self-orient toward the silicone and the PEOcenter is oriented away from the silicone.

In one embodiment, the hydrophilic polymer has a molecular weight of atleast about 1000 g/mol, 5,000 g/mol, 8,000 g/mol, 10,000 g/mol, or15,000 g/mol. In one embodiment, the hydrophilic polymer comprises botha hydrophilic domain and a partially hydrophobic domain (e.g., acopolymer, also referred to herein as a hydrophobic-hydrophilicpolymer). The hydrophobic domain(s) facilitate the blending of thehydrophilic polymer with the hydrophobic silicone polymer, such as butnot limited to formation of micellar jackets within and/or around thesilicone. In one embodiment, the hydrophobic domain is itself a polymer(i.e., a polymeric hydrophobic domain). For example, in one embodiment,the hydrophobic domain is not a simple molecular head group but israther polymeric. In various embodiments, the molecular weight of anycovalently continuous hydrophobic domain within the hydrophilic polymeris at least about 500 g/mol, 700 g/mol, 1000 g/mol, 2000 g/mol, 5000g/mol, or 8,000 g/mol. In various embodiments, the molecular weight ofany covalently continuous hydrophilic domain within the hydrophilicpolymer is at least about 500 g/mol, 700 g/mol, 1000 g/mol, 2000 g/mol,5000 g/mol, or 8,000 g/mol.

In some embodiments, within a particular layer, the ratio of thesilicone polymer to hydrophilic polymer is selected to provide an amountof oxygen and water-soluble molecule solubility such that oxygen andwater-soluble molecule transport through a domain is optimized accordingto the desired function of that particular layer. Furthermore, in someembodiments, the ratio of silicone polymer to hydrophilic polymer, aswell as the polymeric compositions, are selected such that a layerconstructed from the material has interference characteristics thatinhibit transport of one or more interfering species through the layer.Some known interfering species for a glucose sensor include, but are notlimited to, acetaminophen, ascorbic acid, bilirubin, cholesterol,creatinine, dopamine, ephedrine, ibuprofen, L-dopa, methyldopa,salicylate, tetracycline, tolazamide, tolbutamide, triglycerides, anduric acid. Accordingly, in some embodiments, a siliconepolymer/hydrophilic polymer layer as disclosed herein is less permeableto one or more of these interfering species than to the analyte, e.g.,glucose.

In some of these embodiments, the ratio of silicone polymer tohydrophilic polymer (in the layers incorporating the blends) variesaccording to the desired functionality of each layer. The relativeamounts of silicone polymer and hydrophilic polymer described below arebased on the respective amounts found in the cured polymeric blend. Uponintroduction into an aqueous environment, some of the polymericcomponents may leach out, thereby changing the relative amounts ofsilicone polymer and hydrophilic polymer. For example, substantialamounts of the portions of the hydrophilic polymer that are notcross-linked may leach out, for example, depending on the hydrophilicpolymer's molecular weight and how tortuous it the diffusion path out ofthe membrane.

In some embodiments, the silicone and hydrophilic polymers form asubstantial blend. Namely, the amount of any cross-linking between thesilicone polymer and the hydrophilic polymer is substantially limited.In various embodiments, at least about 75%, 85%, 95%, or 99% of thesilicone polymer is not covalently linked to the hydrophilic polymer. Insome embodiments, the silicone polymer and the hydrophilic polymer donot cross-link at all unless a cross-linking agent is used (e.g., suchas described below). Similarly, in some embodiments, the amount of anyentanglement (e.g., blending on a molecular level) between the siliconepolymer and the hydrophilic polymer is substantially limited. In oneembodiment, the silicone polymer and hydrophilic polymers formmicrodomains. For example, in one embodiment, the silicone polymer formsmicellar jacket structures surrounded by a network of hydrophilicpolymer.

The silicone polymer for use in the silicone/hydrophilic polymer blendmay be any suitable silicone polymer. In some embodiments, the siliconepolymer is a liquid silicone rubber that may be vulcanized using ametal- (e.g., platinum), peroxide-, heat-, ultraviolet-, or otherradiation-catalyzed process. In some embodiments, the silicone polymeris a dimethyl- and methylhydrogen-siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen-siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

The hydrophilic polymer for use in the blend may be any suitablehydrophilic polymer, including but not limited to components such aspolyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein byreference). In one embodiment, the hydrophilic polymer is a copolymer ofpoly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). Suitablesuch polymers include, but are not limited to, PEO-PPO diblockcopolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblockcopolymers, alternating block copolymers of PEO-PPO, random copolymersof ethylene oxide and propylene oxide, and blends thereof. In someembodiments, the copolymers may be optionally substituted with hydroxysubstituents. Commercially available examples of PEO and PPO copolymersinclude the PLURONIC® brand of polymers available from BASF®. SomePLURONIC® polymers are triblock copolymers of poly(ethyleneoxide)-poly(propylene oxide)-poly(ethylene oxide) having the generalmolecular structure:HO—(CH₂CH₂O)_(x)—(CH₂CH₂CH₂O)_(y)—(CH₂CH₂O)_(x)—OHwhere the repeat units x and y vary between various PLURONIC® products.The poly(ethylene oxide) blocks act as a hydrophilic domain allowing thedissolution of aqueous agents in the polymer. The poly(propylene oxide)block acts as a hydrophobic domain facilitating the blending of thePLURONIC® polymer with a silicone polymer. In one embodiment, PLURONIC®F-127 is used having x of approximately 100 and y of approximately 65.The molecular weight of PLURONIC® F-127 is approximately 12,600 g/mol asreported by the manufacture. Other PLURONIC® polymers includePPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Othersuitable commercial polymers include, but are not limited to,SYNPERONICS® products available from UNIQEMA®.

The polyether structure of PLURONIC® polymers is relatively inert.Accordingly, without being bound by any particular theory, it isbelieved that the PLURONIC® polymers do not substantially react with thecomponents in MED-4840 or other silicone polymer precursors.

Those of skill in the art will appreciate that other copolymers havinghydrophilic and hydrophobic domains may be used. For example, in onealternative embodiment, a triblock copolymer having the structurehydrophobic-hydrophilic-hydrophobic may be used. In another alternativeembodiment, a diblock copolymer having the structurehydrophilic-hydrophobic is used. Additional devices, methods andcompositions can be found in U.S. Patent Publication No.US-2006-0270923-A1, and U.S. patent application Ser. No. 11/404,417,filed on Apr. 14, 2006, and entitled “SILICONE BASED MEMBRANES FOR USEIN IMPLANTABLE GLUCOSE SENSORS,” both of which are incorporated hereinby reference.

Layers and/or domains that include a silicone polymer-hydrophilicpolymer blend can be made using any of the methods of forming polymerblends known in the art. In one embodiment, a silicone polymer precursor(e.g., MED-4840) is mixed with a solution of a hydrophilic polymer(e.g., PLURONIC® F-127 dissolved in a suitable solvent such as acetone,ethyl alcohol, or 2-butanone). The mixture may then be drawn into a filmor applied in a multi-layer membrane structure using any method known inthe art (e.g., spraying, painting, dip coating, vapor depositing,molding, 3-D printing, lithographic techniques (e.g., photolithograph),micro- and nano-pipetting printing techniques, etc.). The mixture maythen be cured under high temperature (e.g., 50-150° C.). Other suitablecuring methods include ultraviolet or gamma radiation, for example.During curing, the silicone polymer precursor will vulcanize and thesolvent will evaporate. In one embodiment, after the mixture is drawninto a film, another preformed layer of the membrane system is placed onthe film. Curing of the film then provides bonding between the film andthe other preformed layer. In one embodiment, the preformed layer is thecell disruptive layer. In one embodiment, the cell disruptive domaincomprises a preformed porous silicone membrane. In other embodiments,the cell disruptive domain is also formed from a siliconepolymer/hydrophilic polymer blend. In some embodiments, multiple filmsare applied on top of the preformed layer. Each film may posses a finiteinterface with adjacent films or may together form a physicallycontinuous structure having a gradient in chemical composition.

Some amount of cross-linking agent may also be included in the mixtureto induce cross-linking between hydrophilic polymer molecules. Forexample, when using a PLURONIC® polymer, a cross-linking system thatreacts with pendant or terminal hydroxy groups or methylene, ethylene,or propylene hydrogen atoms may be used to induce cross linking.Non-limiting examples of suitable cross-linking agents include ethyleneglycol diglycidyl ether (EGDE), poly(ethylene glycol) diglycidyl ether(PEGDE), or dicumyl peroxide (DCP). While not being bound by anyparticular theory, at low concentrations, these cross-linking agents arebelieved to react primarily with the PLURONIC® polymer with some amountpossibly inducing cross-linking in the silicone polymer or between thePLURONIC® polymer and the silicone polymer. In one embodiment, enoughcross-linking agent is added such that the ratio of cross-linking agentmolecules to hydrophilic polymer molecules added when synthesizing theblend is about 10 to about 30 (e.g., about 15 to about 20). In oneembodiment, from about 0.5% to about 15% w/w of cross-linking agent isadded relative to the total dry weights of cross-linking agent, siliconepolymer, and hydrophilic polymer added when blending the ingredients (inone example, from about 1% to about 10%). In one embodiment, from about5% to about 30% of the dry ingredient weight is the PLURONIC® polymer.During the curing process, substantially all of the cross-linking agentis believed to react, leaving substantially no detectable unreactedcross-linking agent in the final film.

In some embodiments, other agents may be added to the mixture tofacilitate formation of the blend. For example, a small amount ofbutylhydroxy toluene (BHT) (e.g., about 0.01% w/w) or other suitableantioxidant may be mixed with a PLURONIC® to stabilize it.

In some alternative embodiments, precursors of both the silicone polymerand hydrophilic polymer may be mixed prior to curing such thatpolymerization of both the silicone polymer and the hydrophilic polymeroccur during curing. In another embodiment, already polymerized siliconepolymer is mixed with a hydrophilic polymer such that no significantpolymerization occurs during curing.

While not wishing to be bound by theory, it is believed that amicelle-like structure, referred to herein as a micellar jacketstructure, can be formed by combining certain hydrophobic polymers(e.g., silicone) with certain amphipathic polymers (e.g., hydrophilicpolymers such as PLURONIC® polymers), which, when substantially blended,create a mechanism by which glucose and other analytes are transportedat a limited rate. One example of a limited rate is diffusion of oxygenand glucose into the membrane at a ratio of 50:1 (50 oxygen moleculesfor every one glucose molecule). In a preferred embodiment, oxygen andglucose diffuse into the membrane at the limited rate of 100:1. In amore preferred embodiment, oxygen and glucose diffuse into the membraneat the limited rate of 200:1.

In a first mechanism of limited analyte transport, it is believed thatthe PLURONIC® hydrophilic and hydrophobic constituents can promoteself-organization of the PLURONIC® molecules, in conjunction with thesilicone, into micellar jackets. The micellar jackets provide acontiguous channel (e.g., a tortuous path) though the silicone, throughwhich the analyte travels. For example, at a first side of amembrane/domain, glucose dissolves into the hydrophilic component of themicellar jackets (e.g., within the membrane/domain) and diffuses throughthe hydrophilic portion of adjacent micellar jackets, to reach theopposite side of the membrane/domain.

In a second mechanism of limited analyte transport, it is believed thatmicellar jackets can provide a hydrophilic phase within the siliconemembrane/domain structure. There is an energetic barrier to diffusion ofthe analyte (e.g., glucose) into the silicone. However, an energetic,thermodynamic force (e.g., an analyte concentration gradient) drives theanalyte to pass across/through the membrane by “jumping” from onemicellar jacket to another. For example, a glucose concentrationgradient can provide the energy for a glucose molecule to pass into themembrane domain or layer (e.g., the cell impermeable domain formed of asubstantial blend of silicone and PLURONIC®), to the first micellarjacket, then to “jump” to the next micellar jacket, and so on, until themolecule reaches the opposite side of the membrane domain/layer.

In one exemplary embodiment, a silicone-hydrophilic polymer (e.g.,wherein the hydrophilic polymer is an amphipathic polymer, such as butnot limited to PLURONIC®) blend is believed to promote themacromolecular self-organization of micellar jackets that clothecolloidal silicone globules (e.g., silicone granules that form athree-dimensional contiguous macromolecular structure havingsilicone-to-silicone contacts between the silicone granules, coated withthe hydrophilic polymer), within the membrane domain. The hydrophilicgroups of the micellar jackets orient toward the silicone, with thehydrophobic portions of the polymer oriented away from the silicone coreof the structure. For example, in the case of silicone globules clothedwith PLURONIC® (PEO-PPO-PEO), it is believed that it isthermodynamically favorable for a PLURONIC® molecule to orient itselfsuch that the PPO “lies against” the silicone and the PEO to bends awayfrom the silicone, for example, in a U-like shape. Inverse micellarjackets are also possible, for example, inverted micellar jackets (e.g.,with the hydrophobic PPO facing outward toward the silicone and thehydrophilic PEO facing inward) within the silicone. Additionally, themicellar jackets may not be in direct, physical contact with each other,which would provide a thermodynamic barrier to molecules entering themembrane layer and traveling through/across the layer by energetically“jumping” from one micellar jacket to the next.

In addition to facilitating analyte passage through the membrane domain,it has been found that the micellar jacket structure blocks diffusion ofsmall, reactive oxygen and nitrogen interferents (e.g., H₂O₂, oxygenradicals, peroxynitrates, etc.) that can cause non-constant noise. Whilenot wishing to be bound by theory, it is believed that the micellarjacket structure sufficiently slows the diffusion of the reactive oxygenand nitrogen interferents such that these molecules self-annihilatebefore reaching the electroactive surface(s). In contrast, it isbelieved that large molecular weight interferents (e.g., acetaminophenand ascorbate) are sterically and/or thermodynamically blocked and/ortrapped by the micellar jackets, and thus do not reach the electroactivesurface(s). Accordingly, non-constant noise produced by both small andlarge molecular weight interferents is attenuated, with improved sensorfunction as a result.

In one exemplary embodiment, the sensor is configured to blocknon-constant, non-analyte-related noise and comprises first and secondworking electrodes separated by a spacer (as described elsewhere herein,see FIGS. 1B-1C or 7F-7G) and a membrane system (e.g., FIGS. 2A-2B)comprising at least an enzyme domain and a silicone-hydrophilic polymerblend that includes a micellar jacket structure. In some furtherembodiments, the membrane system includes at least one additionaldomain, such as but not limited to an electrode domain, an interferencedomain, a resistance domain and a cell disruptive domain (e.g., a mostdistal domain comprising a silicone-hydrophilic polymer blend and amicellar jacket structure). In one preferred embodiment, the membranesystem includes a combined resistance-interference domain formed of asilicone-hydrophilic polymer blend having a micellar jacket structure,whereby the combined silicone-hydrophilic polymer blendresistance-interference domain can modulate the flux of the analyte intothe membrane system and reduce noise by blocking the passage of at leastone interferent (e.g., acetaminophen) into the membrane system. In afurther preferred embodiment, the sensor is configured fortranscutaneous insertion into the host. In another further preferredembodiment, the sensor is configured for insertion into a host's vessel,such as but not limited to a blood vessel.

Fluid Pocket Formation to Reduce Noise

While not wishing to be bound by theory, it is believed thatnon-constant, non-analyte-related noise can be decreased by dilutingand/or removing interferents, such as by increasing fluid bulk (e.g., afluid pocket), increasing bulk fluid flow and/or increasing diffusionrate around at least a portion of the sensor, such as the sensingportion of the sensor. Furthermore, a physical spacer can reduce theeffect of lymph pooling (e.g., build-up of interferents in the tissuesurrounding an implanted sensor) due to local compression (describedelsewhere herein) by mechanically maintaining a fluid pocket. Since aspacer can maintain the fluid bulk around the sensor during localcompression, the effect of interferant concentration increases can besuppressed or reduced, thereby reducing noise and promoting optimalsensor function. One preferred embodiment provides a device with reducednoise (e.g., during intermittent host sedentary periods) having anarchitecture that allows and/or promotes increased fluid bulk and/orincreased bulk fluid flow in the area surrounding at least a portion ofan implanted sensor in vivo.

A variety of structures can be incorporated into the sensorconfiguration as an interference domain to allow and/or promoteincreased (e.g., to stimulate or to promote) fluid bulk, bulk fluidflow, and/or diffusion rate, such as by forming a fluid pocket, whichcan reduce noise. These structures can include but are not limited tospacers, meshes, shedding layers, roughened surfaces, machineablematerials, nanoporous materials, shape-memory materials, porous memorymaterials, self-assembly materials, collapsible materials, biodegradablematerials, combinations thereof, and the like. Structures that promoteincreased fluid bulk and/or increased bulk fluid flow can also includebut are not limited to structures that promote fluid influx or efflux(e.g., fluid influx-promoting architecture, fluid efflux-promotingarchitecture), that promote vasodilation (e.g., vasodilatingarchitecture), that promote inflammation (e.g., inflammatoryarchitecture), that promote wound healing or perpetuate wounding (e.g.,wound-healing architecture and wounding architecture, respectively),that promote angiogenesis (e.g., angiogenic architecture), that suppressinflammation (e.g., an anti-inflammatory architecture) or combinationsthereof.

In certain embodiments, the device includes a physical spacer betweenthe sensor and the surrounding tissue. A spacer allows for a liquidsheath to form around at least a portion of the sensor, such as the areasurrounding the electrodes, for example. A fluid sheath can provide afluid bulk that dilutes or buffers interferants while promoting glucoseand oxygen transport to the sensor.

In some embodiments, the spacer is a mesh or optionally a fibrousstructure. Suitable mesh materials are known in the art and includeopen-weave meshes fabricated of biocompatible materials such as but notlimited to PLA, PGA, PP, nylon and the like. Mesh spacers can be applieddirectly to the sensing mechanism or over a biointerface membrane, suchas a porous biointerface membrane disclosed elsewhere herein. Meshspacers can act as a fluid influx- or efflux-promoting structure andprovides the advantage of relatively more rapid fluid movement, mixingand/or diffusion within the mesh to reduce local interferantconcentrations and increasing glucose and oxygen concentrations. Theincreased fluid volume within the mesh can also promote increased fluidmovement in and out of the area, which brings in glucose and oxygenwhile removing or diluting interferants.

In one exemplary embodiment, the sensor is wrapped with a single layerof open weave polypropylene (PP) biocompatible mesh. When the sensor isinserted, the mesh holds the surrounding tissue away from the sensorsurface and allows an influx of extracellular fluid to enter the spaceswithin the mesh, thereby creating a fluid pocket around the sensor.Within the fluid pocket, fluid can mix substantially rapidly asextracellular fluid enters and leaves the fluid pocket or due to hostmovement. Interferants are carried by the fluid and therefore can bemixed and/or diluted. Since the host can wear the sensor for a pluralityof days, sedentary periods will inevitably occur. During these periodsinterferants can accumulate. However, the increased fluid volumeprovided by the mesh can substantially buffer accumulated interferantsuntil the sedentary period ends. When the sedentary period is over, anyaccumulated interferants can be diluted or carried away by an influx orefflux of fluid.

In an alternative embodiment, a mesh can be applied to a sensor eithersymmetrically or asymmetrically. For example, the mesh can be tightlywrapped around the sensor. In another example, a strip of mesh can beapplied to only one side of the sensor. In yet another example, the meshcan form a flat envelope about a few millimeters to about a centimeterwide, with the sensor sandwiched within the envelope. In someembodiments, the mesh can cover only a portion of the sensor, such asthe portion containing the electrochemically reactive surface(s). Inother embodiments, the mesh can cover the entire sensor.

In another alternative embodiment, noise can be reduced by inclusion ofa hydrogel on the surface of at least a portion of the sensor, such asthe sensing region. A hydrogel is a network of super absorbent (they cancontain 20%-99% or weight % water, preferably 80% to over 99% weight %water) natural or synthetic polymer chains. Hydrogels are sometimesfound as a colloidal gel in which water is the dispersion medium. Sincehydrogels are nonporous, fluid and interferants within the hydrogel moveby diffusion. Accordingly, the movement of molecules within hydrogels isrelatively slower than that possible within mesh-based fluid pockets asdescribed above. Optionally, the hydrogel can be biodegradable. Abiodegradable hydrogel can provide a fluid pocket that graduallydiminishes and is eventually eliminated by the surrounding tissue.

In a further embodiment, a hydrogel includes a flexible,water-swellable, film (as disclosed elsewhere herein) having a “dryfilm” thickness of from about 0.05 micron or less to about 20 microns ormore, more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35,0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 to about 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferablyfrom about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. “Dryfilm” thickness refers to the thickness of a cured film cast from acoating formulation by standard coating techniques. The hydrogelmaterial can be applied to the entire sensor or a portion of it, usingany method known in the art, such as but not limited to dipping,painting, spraying, wrapping, and the like.

In certain embodiments, scavenging agents (e.g., bioactive agents thatcan scavenge, bind-up or substantially inactivate interferants) can beincorporated into the hydrogel or other aspect of the device (e.g.,membrane system). Scavenging agents can suppress prolonged wounding andinflammation by removing signal associated with irritating substancesfrom the locality of the sensor and/or internally generated hydrogenperoxide.

One exemplary scavenging agent embodiment incorporates an H₂O₂-degradingenzyme, such as but not limited to glutathione peroxidase (GSHperoxidase), heme-containing peroxidases, eosinophil peroxidase, thyroidperoxidase or horseradish peroxidase (HRP) into the hydrogel to degradethe available H₂O₂ and produce oxygen. The scavenging agent can actwithin the hydrogel or can be released into the local environment to actoutside the hydrogel.

In a further embodiment, a mesh and a hydrogel can be used incombination to provide greater mechanical support (to hold thesurrounding tissue away from the sensor) while slowing down thediffusion rate within the mesh-hydrogel layer. For example, a PP meshcan be applied to the sensor followed by spraying a dry hydrogelmaterial onto the PP-wrapped sensor. Alternatively, the hydrogel can bedried within the mesh before application to the sensor. Upon sensorimplantation, the hydrogel can absorb fluid from the surrounding tissue,expand and fill the mesh pores. In a further example, the hydrogel canbe biodegradable. In this example, the hydrogel can initially slow fluidmovement. But as the hydrogel is biodegraded, the pores of the mesh areopened up and fluid movement can speed up or increase.

A variety of alternative materials can be used to create architecturesthat create a fluid pocket. For example, shape-memory materials can beused as an alternative to a mesh, to form a fluid pocket around thesensor. Shape-memory materials are metals or polymers that “remember”their geometries. Shape-memory metals (e.g., memory metals or smartwire) include copper-zinc-aluminum, copper-aluminum-nickel, andnickel-titanium (NiTi) alloys. Shape-memory polymers include materialssuch as polynorbornene, segmented poly(epsilon-caprolactone)polyurethanes, poly(ethylene glycol)-poly(epsilon-caprolactone) diblockcopolymers, and the like, for example. A shape-memory material can bedeformed from its “original” conformation and regains its originalgeometry by itself in response to a force, such as temperature orpressure.

In one embodiment, a porous memory material that has been collapsed intoa flat, nonporous sheet can be applied to the exterior of the sensor asa flat film. After insertion into the body, increased temperature ormoisture exposure can stimulate the memory material to transform to a3-dimensional, porous architecture that promotes fluid pocket formation,for example.

In an alternative embodiment, nanoporous materials, which act asmolecular sieves, can be used to exclude interferants surrounding thesensor. In another alternative embodiment, a swellable material (e.g., amaterial having an initial volume that absorbs fluid, such as water,when it contacts the fluid to become a second volume that is greaterthan the initial volume) or collapsible material (e.g., a materialhaving an initial volume that collapse to a second volume that issmaller than the initial volume) can produce or maintain a fluid pocket.

In yet another embodiment, materials with differing characteristics canbe applied in combination, such as alternating bands or layers, tosuppress uniform capsule formation. For example, alternating bands ofcollapsible and non-collapsible swellable material can be applied arounda portion of the sensor. Upon implantation, both materials swell withfluid from the surrounding tissue. However, only the segments ofcollapsible material can deform. Since the material surrounding thesensor will be irregular, it can disrupt formation of a continuous celllayer, thereby reducing noise and extending sensor life.

In addition to providing a physical spacer, mesh, porous material or thelike, irritating sensor configurations can reduce noise by promotingfluid pocket formation and/or increased bulk fluid flow. Accordingly,one embodiment of an irritating biointerface includes a structure havinga roughened surface, which can rub or poke adjacent cells in vivo. Thesensor surface can be roughened by coating the sensor with a machineablematerial that is or can be etched to form ridges, bristles, spikes,grids, grooves, circles, spirals, dots, bumps, pits or the like, forexample. The material can be any convenient, biocompatible material,such as machined porous structures that are overlaid on the sensor, suchas but not limited to machineable metal matrix composites, bonesubstrates such as hydroxyapatite, coral hydroxyapatite and β-tricalciumphosphate (TCP), porous titanium (Ti) mixtures made by sintering ofelemental powders, bioglasses (calcium and silicon-based porous glass),ceramics and the like. The material can be “machined” by any convenientmeans, such as but not limited to scraping, etching, lathing orlasering, for example.

Micro-motion of the sensor can increase the irritating effect of aroughened surface. Micro-motion is an inherent property of any implanteddevice, such as an implanted glucose sensor. Micro-motion of the device(e.g., minute movements of the device within the host) is caused by hostmovements, ranging from breathing and small local muscle movements togross motor movements, such as walking, running or even getting up andsitting down. External forces, such as external pressure application,can also cause micro-motion. Micro-motion includes movement of thesensor back and forth, rotation, twisting and/or turning. Accordingly,as the sensor is moved by micro-motion, the sensor's rough surface canrub more vigorously against the surrounding tissue, causing increased orextended wounding, resulting in additional stimulation of the woundhealing process and increases in fluid bulk, bulk fluid flow and/orfluid pocket formation, with a concomitant reduction in noise.

In another embodiment, an irritating architecture is formed fromself-assembly materials. Self-assembly biomaterials comprise specificpolypeptides that are designed a priori to self-assemble into targetednano- and microscopic structures. Intramolecular self-assemblingmolecules are often complex polymers with the ability to assemble fromthe random coil conformation into a well-defined stable structure(secondary and tertiary structure). A variety of self-assembly materialsknown in the art can find use in the present embodiment. For example,PuraMatrix™ (3DM Inc., Cambridge, Mass., USA) can be used to createsynthetic self-assembling peptide nanofiber scaffolds and defined 3-Dmicroenvironments.

In an exemplary embodiment of an irritating biointerface, an irritatingsuperstructure is applied to the working electrode or the completedsensor. A “superstructure,” as used herein is a broad term and used inits ordinary sense, including, without limitation, to refer to anystructure built on something else, such as but not limited to theoverlying portion of a structure. An irritating superstructure caninclude any substantial structure that prevents cell attachment and isirritating to the surrounding tissue in vivo. In one example, anirritating superstructure can include large spaces, such as at leastabout 50 μm wide and at least about 50 μm deep. Cells surrounding thesensor can be prevented from attachment in the spaces within thesuperstructure, allowing fluid to fill these spaces. In some exemplaryembodiments, an irritating superstructure takes advantage of sensormicromotion, to prevent cell attachment and stimulate fluid pocketformation.

In one exemplary embodiment, an irritating superstructure is comprisedof ridges at least about 0.25 to 0.50 μm in diameter and about 50 μmhigh, and separated by at least about 0.25 to 0.50 μm. In anotherexemplary embodiment, an exposed silver wire, at least about 0.25 to0.50 μm in diameter, is applied to the sensor exterior to form groovesabout 50 μm wide and about 50 μm deep. Since silver is pro-inflammatoryand stimulates fluid influx from the surrounding tissues, thecombination of an irritating superstructure and a chemical irritantcould promote an increased rate of fluid influx or prolong irritationand fluid influx. In yet another exemplary embodiment, with reference tothe embodiment shown in FIG. 1C, the configuration (e.g., diameter) ofthe reference electrode 20 can be changed (e.g., increased in sizeand/or coil spacing) such that the reference electrode, itself, becomesan irritating superstructure, with or without a coating 22 as disclosedelsewhere herein.

Porous Membrane

In addition to the devices described above, fluid bulk and or bulk fluidflow at and/or adjacent to the sensor can be increased by incorporatinga porous membrane into the sensor system, such that noise issubstantially reduced and sensor accuracy and/or sensitivity areimproved. A porous membrane can be referred to as a “bioprotectivedomain” or a “cell disruptive domain.” In some embodiments, the sensorincludes a porous material disposed over some portion thereof, whichmodifies the host's tissue response to the sensor and thereby reducesnoise (e.g., due to a local build up of interferents). For example, insome embodiments, the porous material surrounding the sensoradvantageously enhances and extends sensor performance and lifetime inthe short-term by slowing or reducing cellular migration to the sensorand associated degradation that would otherwise be caused by cellularinvasion if the sensor were directly exposed to the in vivo environment.Alternatively, the porous material can provide stabilization of thesensor via tissue ingrowth into the porous material in the long-term.Suitable porous materials include silicone, polytetrafluoroethylene,expanded polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene,homopolymers, copolymers, terpolymers of polyurethanes, polypropylene(PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polyvinylalcohol (PVA), polybutylene terephthalate (PBT), polymethylmethacrylate(PMMA), polyether ether ketone (PEEK), polyamides, polyurethanes,cellulosic polymers, poly(ethylene oxide), poly(propylene oxide) andcopolymers and blends thereof, polysulfones and block copolymers thereofincluding, for example, di-block, tri-block, alternating, random andgraft copolymers, as well as metals, ceramics, cellulose, hydrogelpolymers, poly (2-hydroxyethyl methacrylate, pHEMA), hydroxyethylmethacrylate, (HEMA), polyacrylonitrile-polyvinyl chloride (PAN-PVC),high density polyethylene, acrylic copolymers, nylon, polyvinyldifluoride, polyanhydrides, poly(l-lysine), poly (L-lactic acid),hydroxyethylmethacrylate, hydroxyapeptite, alumina, zirconia, carbonfiber, aluminum, calcium phosphate, titanium, titanium alloy, nintinol,stainless steel, and CoCr alloy, or the like, such as are described inU.S. Patent Publication No. US-2005-0031689-A1 and U.S. PatentPublication No. US-2005-0112169-A1.

In some embodiments, the porous material surrounding the sensor providesunique advantages in the short-term (e.g. one to 14 days) that can beused to enhance and extend sensor performance and lifetime. However,such materials can also provide advantages in the long-term too (e.g.greater than 14 days). Particularly, the in vivo portion of the sensor(the portion of the sensor that is implanted into the host's tissue) isencased (partially or fully) in a porous material. The porous materialcan be wrapped around the sensor (for example, by wrapping the porousmaterial around the sensor or by inserting the sensor into a section ofporous material sized to receive the sensor). Alternately, the porousmaterial can be deposited on the sensor (for example, by electrospinningof a polymer directly thereon). In yet other alternative embodiments,the sensor is inserted into a selected section of porous biomaterial.Other methods for surrounding the in vivo portion of the sensor with aporous material can also be used as is appreciated by one skilled in theart.

The porous material surrounding the sensor advantageously slows orreduces cellular migration to the sensor and associated degradation thatwould otherwise be caused by cellular invasion if the sensor weredirectly exposed to the in vivo environment. Namely, the porous materialprovides a barrier that makes the migration of cells towards the sensormore tortuous and therefore slower (providing short-term advantages). Itis believed that this reduces or slows the sensitivity loss normallyobserved in a short-term sensor over time.

In an embodiment wherein the porous material is a high oxygen solubilitymaterial, such as porous silicone, the high oxygen solubility porousmaterial surrounds some of or the entire in vivo portion of the sensor.In some embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubledomain (for example, a silicone- or fluorocarbon-based material) toenhance the supply/transport of oxygen to the enzyme membrane and/orelectroactive surfaces. It is believed that some signal noise normallyseen by a conventional sensor can be attributed to an oxygen deficit.Silicone has high oxygen permeability, thus promoting oxygen transportto the enzyme layer. By enhancing the oxygen supply through the use of asilicone composition, for example, glucose concentration can be less ofa limiting factor. In other words, if more oxygen is supplied to theenzyme and/or electroactive surfaces, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.While not being bound by any particular theory, it is believed thatsilicone materials provide enhanced bio-stability when compared to otherpolymeric materials such as polyurethane.

In certain aspects, modifying a sensor with a biointerface structure,material, matrix, and/or membrane that creates a space appropriate forfilling with fluid in vivo can enhance sensor performance. In someembodiments, the sensor includes a porous biointerface material, whichallows fluid from the surrounding tissues to form a fluid-filled pocketaround at least a portion of the sensor. It is believed that thefluid-filled pocket provides a sufficient source of analyte-containingfluid for accurate sensor measurement in the short-term. Additionally oralternatively, inclusion of bioactive agents can modify the host'stissue response, for example to reduce or eliminate tissue ingrowth orother cellular responses into the biointerface.

In some aspects, modifying a sensor with a structure, material, and/ormembrane/matrix that allows tissue ingrowth without barrier cellformation can enhance sensor performance. For example, a vascularizedbed of tissue for long-term analyte sensor measurement. In someembodiments, a porous biointerface membrane, including a plurality ofinterconnected cavities and a solid portion, covering at least thesensing portion of a sensor allows vascularized tissue ingrowth therein.Vascularized tissue ingrowth provides a sufficient source ofanalyte-containing tissue in the long-term. Additionally oralternatively, inclusion of bioactive agents can modify the host'stissue response, for example to reduce or eliminate barrier cell layerformation within the membrane.

When used herein, the terms “membrane” and “matrix” are meant to beinterchangeable. In these embodiments first domain is provided thatincludes an architecture, including cavity size, configuration, and/oroverall thickness, that modifies the host's tissue response, forexample, by creating a fluid pocket, encouraging vascularized tissueingrowth, disrupting downward tissue contracture, resisting fibroustissue growth adjacent to the device, and/or discouraging barrier cellformation. The biointerface preferably covers at least the sensingmechanism of the sensor and can be of any shape or size, includinguniform, asymmetrically, or axi-symmetrically covering or surrounding asensing mechanism or sensor.

In some embodiments, a second domain is optionally provided that isimpermeable to cells and/or cell processes. A bioactive agent isoptionally provided that is incorporated into the at least one of thefirst domain, the second domain, the sensing membrane, or other part ofthe implantable device, wherein the bioactive agent is configured tomodify a host tissue response.

In one embodiment, a porous material that results in increased fluidbulk, bulk fluid flow and/or diffusion rate, as well as formation ofclose vascular structures, is a porous polymer membrane, such as but notlimited to polytetrafluoroethylene (PTFE), polysulfone, polyvinylidenedifluoride, polyacrylonitrile, silicone, polytetrafluoroethylene,expanded polytetrafluoroethylene, polyethylene-co-tetrafluoroethylene,polyolefin, polyester, polycarbonate, biostable polytetrafluoroethylene,homopolymers, copolymers, terpolymers of polyurethanes, polypropylene(PP), polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polyvinylalcohol (PVA), polybutylene terephthalate (PBT), polymethylmethacrylate(PMMA), polyether ether ketone (PEEK), polyamides, polyurethanes,cellulosic polymers, poly(ethylene oxide), poly(propylene oxide) andcopolymers and blends thereof, polysulfones and block copolymers thereofincluding, for example, di-block, tri-block, alternating, random andgraft copolymers, as well as metals, ceramics, cellulose, hydrogelpolymers, poly (2-hydroxyethyl methacrylate, pHEMA), hydroxyethylmethacrylate, (HEMA), polyacrylonitrile-polyvinyl chloride (PAN-PVC),high density polyethylene, acrylic copolymers, nylon, polyvinyldifluoride, polyanhydrides, poly(l-lysine), poly (L-lactic acid), andhydroxyethylmethacrylate, having an average nominal pore size of atleast about 0.6 to 20 μm, using conventional methods for determinationof pore size in the trade. In one embodiment, at least approximately 50%of the pores of the membrane have an average size of approximately 0.6to about 20 μm, such as described in U.S. Pat. No. 5,882,354. In thisexemplary embodiment, the structural elements, which provide thethree-dimensional conformation, can include fibers, strands, globules,cones or rods of amorphous or uniform geometry that is smooth or rough.These elements, hereafter referred to as “strands,” have in general onedimension larger than the other two and the smaller dimensions do notexceed five microns.

In another further embodiment, the porous polymer membrane material, asdescribed above, consists of strands that define “apertures” formed by aframe of the interconnected strands. The apertures have an average sizeof no more than about 20 μm in any but the longest dimension. Theapertures of the material form a framework of interconnected apertures,defining “cavities” that are no greater than an average of about 20 μmin any but the longest dimension. In another embodiment the porouspolymer membrane material has at least some apertures having asufficient size to allow at least some vascular structures to be createdwithin the cavities. At least some of these apertures, while allowingvascular structures to form within the cavities, prevent connectivetissue from forming therein because of size restrictions.

In a further embodiment, the porous membrane has frames of elongatedstrands of material that are less than 5 microns in all but the longestdimension and the frames define apertures which interconnect to formthree-dimensional cavities which permit substantially all inflammatorycells migrating into the cavities to maintain a rounded morphology.Additionally, the porous material promotes vascularization adjacent butnot substantially into the porous material upon implantation into ahost. Exemplary materials include but are not limited to polyethylene,polypropylene, polytetrafluoroethylene (PTFE), cellulose acetate,cellulose nitrate, polycarbonate, polyester, nylon, polysulfone, mixedesters of cellulose, polyvinylidene difluoride, silicone,polyacrylonitrile, and the like.

In some embodiments, a short-term sensor is provided with a spaceradapted to provide a fluid pocket between the sensor and the host'stissue. It is believed that this spacer, for example a biointerfacematerial, matrix, mesh, hydrogel and like structures and the resultantfluid pocket provide for oxygen and/or glucose transport to the sensor.

In one exemplary embodiment, the sensor includes a biointerface membraneconfigured to prevent adipose cell contact with an insertedtranscutaneous sensor or an implanted sensor. Preferably, a porousbiointerface membrane surrounds the sensor, covering the sensingmechanism (e.g., at least a working electrode) and is configured to fillwith fluid in vivo, thereby creating a fluid pocket surrounding thesensor. Accordingly, the adipose cells surrounding the sensor are held adistance away (such as the thickness of the porous biointerfacemembrane, for example) from the sensor surface. Accordingly, as theporous biointerface membrane fills with fluid (e.g., creates a fluidpocket), oxygen and glucose are transported to the sensing mechanism inquantities sufficient to maintain accurate sensor function.Additionally, as discussed elsewhere herein, interferants are diluted,suppressing or reducing interference with sensor function.

In another exemplary embodiment, a short-term sensor (or short-termfunction of a long-term sensor) including a biointerface, including butnot limited to, for example, porous biointerface materials, mesh cages,and the like, all of which are described in more detail elsewhereherein, can be employed to improve sensor function in the short-term(e.g., first few hours to days), such as by reducing noise on the sensorsignal. Porous biointerface membranes need not necessarily includeinterconnected cavities for creating a fluid pocket in the short-term.

Bioactive Agents

A variety of bioactive agents are known to promote fluid influx orefflux. Accordingly, incorporation of bioactive agents into the membranecan increasing fluid bulk, bulk fluid flow and/or diffusion rates (andpromoting glucose and oxygen influx), thereby decrease non-constantnoise. In some embodiments, fluid bulk and/or bulk fluid flow areincreased at (e.g., adjacent to the sensor exterior surface) the sensorby incorporation of one or more bioactive agents. In some embodiments,the sensor is configured to include a bioactive agent that irritates thewound and stimulates the release of soluble mediators that are known tocause a local fluid influx at the wound site. In some embodiments, thesensor is configured to include a vasodilating bioactive agent, whichcan cause a local influx of fluid from the vasculature.

A variety of bioactive agents can be found useful in preferredembodiments. Exemplary bioactive agents include but are not limited toblood-brain barrier disruptive agents and vasodilating agents,vasodilating agents, angiogenic factors, and the like. Useful bioactiveagents include but are not limited to mannitol, sodium thiosulfate,VEGF/VPF, NO, NO-donors, leptin, bradykinin, histamines, bloodcomponents, platelet rich plasma (PRP), matrix metalloproteinases (MMP),Basic Fibroblast Growth Factor (bFGF), (also known as Heparin BindingGrowth Factor-II and Fibroblast Growth Factor II), Acidic FibroblastGrowth Factor (aFGF), (also known as Heparin Binding Growth Factor-I andFibroblast Growth Factor-I), Vascular Endothelial Growth Factor (VEGF),Platelet Derived Endothelial Cell Growth Factor BB (PDEGF-BB),Angiopoietin-1, Transforming Growth Factor Beta (TGF-Beta), TransformingGrowth Factor Alpha (TGF-Alpha), Hepatocyte Growth Factor, TumorNecrosis Factor-Alpha (TNF-Alpha), Placental Growth Factor (PLGF),Angiogenin, Interleukin-8 (IL-8), Hypoxia Inducible Factor-I (HIF-1),Angiotensin-Converting Enzyme (ACE) Inhibitor Quinaprilat, Angiotropin,Thrombospondin, Peptide KGHK, Low Oxygen Tension, Lactic Acid, Insulin,Leptin, Copper Sulphate, Estradiol, prostaglandins, cox inhibitors,endothelial cell binding agents (for example, decorin or vimentin),glenipin, hydrogen peroxide, nicotine, and Growth Hormone. Still otheruseful bioactive agents include enzymes, cytotoxic or necrosing agents(e.g., pactataxyl, actinomycin, doxorubicin, daunorubicin, epirubicin,bleomycin, plicamycin, mitomycin), cyclophosphamide, chlorambucil,uramustine, melphalan, bryostatins, inflammatory bacterial cell wallcomponents, histamines, pro-inflammatory factors and the like.

Bioactive agents can be added during manufacture of the sensor byincorporating the desired bioactive agent in the manufacturing materialfor one or more sensor layers or into an exterior biomaterial, such as aporous silicone membrane. For example, bioactive agents can be mixedwith a solution during membrane formation, which is subsequently appliedonto the sensor during manufacture. Alternatively, the completed sensorcan be dipped into or sprayed with a solution of a bioactive agent, forexample. The amount of bioactive agent can be controlled by varying itsconcentration, varying the indwell time during dipping, applyingmultiple layers until a desired thickness is reached, and the like, asdisclosed elsewhere herein. In an alternative embodiment, the bioactiveagent is microencapsulated before application to the sensor. Forexample, microencapsulated bioactive agent can be sprayed onto acompleted sensor or incorporated into a structure, such as an outer meshlayer or a shedding layer. Microencapsulation can offer increasedflexibility in controlling bioactive agent release rate, time of releaseoccurrence and/or release duration.

Chemical systems/methods of irritation can be incorporated into anexterior sensor structure, such as the biointerface membrane (describedelsewhere herein) or a shedding layer that releases the irritating agentinto the local environment. For example, in some embodiments, a“shedding layer” releases (e.g., sheds or leaches) molecules into thelocal vicinity of the sensor and can speed up osmotic fluid shifts. Insome embodiments, a shedding layer can provide a mild irritation andencourage a mild inflammatory/foreign body response, thereby preventingcells from stabilizing and building up an ordered, fibrous capsule andpromoting fluid pocket formation.

A shedding layer can be constructed of any convenient, biocompatiblematerial, include but not limited to hydrophilic, degradable materialssuch as polyvinylalcohol (PVA), PGC, Polyethylene oxide (PEO),polyethylene glycol-polyvinylpyrrolidone (PEG-PVP) blends, PEG-sucroseblends, hydrogels such as polyhydroxyethyl methacrylate (pHEMA),polymethyl methacrylate (PMMA) or other polymers with quickly degradingester linkages. In certain embodiment, absorbable suture materials,which degrade to compounds with acid residues, can be used. The acidresidues are chemical irritants that stimulate inflammation and woundhealing. In certain embodiments, these compounds include glycolic acidand lactic acid based polymers, polyglactin, polydioxone, polydyconate,poly(dioxanone), poly(trimethylene carbonate) copolymers, and poly(-caprolactone) homopolymers and copolymers, and the like.

In other exemplary embodiments, the shedding layer can be a layer ofmaterials listed elsewhere herein for the first domain, includingcopolymers or blends with hydrophilic polymers such aspolyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers, such as polyethyleneglycol, and block copolymers thereof including, for example, di-block,tri-block, alternating, random and graft copolymers (block copolymersare discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044, herebyincorporated by reference). In one preferred embodiment, the sheddinglayer is comprised of polyurethane and a hydrophilic polymer. Forexample, the hydrophilic polymer can be polyvinylpyrrolidone. In onepreferred embodiment, the shedding layer is polyurethane comprising notless than 5 weight percent polyvinylpyrrolidone and not more than 45weight percent polyvinylpyrrolidone. Preferably, the shedding layercomprises not less than 20 weight percent polyvinylpyrrolidone and notmore than 35 weight percent polyvinylpyrrolidone and, most preferably,polyurethane comprising about 27 weight percent polyvinylpyrrolidone.

In other exemplary embodiments, the shedding layer can include asilicone elastomer, such as a silicone elastomer and a poly(ethyleneoxide) and poly(propylene oxide) co-polymer blend, as disclosed incopending U.S. patent application Ser. No. 11/404,417, filed Apr. 14,2006 and entitled “SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLEGLUCOSE SENSORS.” In one embodiment, the silicone elastomer is adimethyl- and methylhydrogen-siloxane copolymer. In one embodiment, thesilicone elastomer comprises vinyl substituents. In one embodiment, thesilicone elastomer is an elastomer produced by curing a MED-4840mixture. In one embodiment, the copolymer comprises hydroxysubstituents. In one embodiment, the co-polymer is a triblockpoly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) polymer.In one embodiment, the co-polymer is a triblock poly(propyleneoxide)-poly(ethylene oxide)-poly(propylene oxide) polymer. In oneembodiment, the co-polymer is a PLURONIC® polymer. In one embodiment,the co-polymer is PLURONIC® F-127. In one embodiment, at least a portionof the co-polymer is cross-linked. In one embodiment, from about 5% w/wto about 30% w/w of the membrane is the co-polymer.

A shedding layer can take any shape or geometry, symmetrical orasymmetrical, to promote fluid influx in a desired location of thesensor, such as the sensor head or the electrochemically reactivesurfaces, for example. Shedding layers can be located on one side ofsensor or both sides. In another example, the shedding layer can beapplied to only a small portion of the sensor or the entire sensor.

In one exemplary embodiment, a shedding layer comprising polyethyleneoxide (PEO) is applied to the exterior of the sensor, where the tissuesurrounding the sensor can directly access the shedding layer. PEOleaches out of the shedding layer and is ingested by local cells thatrelease pro-inflammatory factors. The pro-inflammatory factors diffusethrough the surrounding tissue and stimulate an inflammation responsethat includes an influx of fluid. Accordingly, early noise can bereduced or eliminated and sensor function can be improved.

In another exemplary embodiment, the shedding layer is applied to thesensor in combination with an outer porous layer, such as a mesh or aporous biointerface as disclosed elsewhere herein. In one embodiment,local cells access the shedding layer through the through pores of aporous silicone biointerface. In one example, the shedding layermaterial is applied to the sensor prior to application of the poroussilicone. In another example, the shedding layer material can beabsorbed into the lower portion of the porous silicone (e.g., theportion of the porous silicone that will be proximal to the sensor afterthe porous silicone has been applied to the sensor) prior to applicationof the porous silicone to the sensor.

Wound Suppression

Non-constant noise can be decreased by wound suppression (e.g., duringsensor insertion), in some embodiments. Wound suppression includes anysystems or methods by which an amount of wounding that occurs uponsensor insertion is reduced and/or eliminated. While not wishing to bebound by theory, it is believed that if wounding is suppressed or atleast significantly reduced, the sensor will be surrounded bysubstantially normal tissue (e.g., tissue that is substantially similarto the tissue prior to sensor insertion). Substantially normal tissue isbelieved to have a lower metabolism than wounded tissue, producing fewerinterferants and reducing early noise.

Wounds can be suppressed or minimized by adaptation of the sensor'sarchitecture to one that either suppresses wounding or promotes rapidhealing, such as an architecture that does not cause substantialwounding (e.g., an architecture configured to prevent wounding), anarchitecture that promotes wound healing, an anti-inflammatoryarchitecture, and the like. In one exemplary embodiment, the sensor isconfigured to have a low profile, a zero-footprint or a smooth surface.For example, the sensor can be formed of substantially thin wires, suchas wires about 50-150 μm in diameter, for example. Preferably, thesensor is small enough to fit within a very small gauge needle, such asa 30, 31, 32, 33, 34, or 35-gauge needle (or smaller) on the Stubsscale, for example. In general, a smaller needle, the more reduces theamount of wounding during insertion. For example, a very small needlecan reduce the amount of tissue disruption and thereby reduce thesubsequent wound healing response. In an alternative embodiment, thesensor's surface is smoothed with a lubricious coating, to reducewounding upon sensor insertion.

Wounding can also be reduced by inclusion of wound-suppressive agents(bioactive agents) that either reduce the amount of initial wounding orsuppress the wound healing process. While not wishing to be bound bytheory, it is believed that application of a wound-suppressing agent,such as an anti-inflammatory, an immunosuppressive agent, ananti-infective agent, or a scavenging agent, to the sensor can create alocally quiescent environment and suppress wound healing. In a quiescentenvironment, bodily processes, such as the increased cellular metabolismassociated with wound healing, can minimally affect the sensor. If thetissue surrounding the sensor is undisturbed, it can continue its normalmetabolism and promote sensor function.

In some embodiment, useful compounds and/or factors for suppressingwounding include but are not limited to first-generation H₁-receptorantagonists: ethylenediamines (e.g., mepyramine (pyrilamine),antazoline), ethanolamines (e.g., diphenhydramine, carbinoxamine,doxylamine, clemastine, and dimenhydrinate), alkylamines (pheniramine,chlorphenamine (chlorpheniramine), dexchlorphenamine, brompheniramine,and triprolidine), piperazines (cyclizine, hydroxyzine, and meclizine),and tricyclics (promethazine, alimemazine (trimeprazine),cyproheptadine, and azatadine); second-generation H₁-receptorantagonists such as acrivastine, astemizole, cetirizine, loratadine,mizolastine, azelastine, levocabastine, and olopatadine; mast cellstabilizers such as cromoglicate (cromolyn) and nedocromil;anti-inflammatory agents, such as acetometaphen, aminosalicylic acid,aspirin, celecoxib, choline magnesium trisalicylate, diclofenacpotassium, diclofenac sodium, diflunisal, etodolac, fenoprofen,flurbiprofen, ibuprofen, indomethacin, interleukin (IL)-10, IL-6 mutein,anti-IL-6 iNOS inhibitors (e.g., L-NMDA), Interferon, ketoprofen,ketorolac, leflunomide, melenamic acid, mycophenolic acid, mizoribine,nabumetone, naproxen, naproxen sodium, oxaprozin, piroxicam, rofecoxib,salsalate, sulindac, and tolmetin; corticosteroids such as cortisone,hydrocortisone, methylprednisolone, prednisone, prednisolone,betamethesone, beclomethasone dipropionate, budesonide, dexamethasonesodium phosphate, flunisolide, fluticasone propionate, paclitaxel,tacrolimus, tranilast, triamcinolone acetonide, betamethasone,fluocinolone, fluocinonide, betamethasone dipropionate, betamethasonevalerate, desonide, desoximetasone, fluocinolone, triamcinolone,triamcinolone acetonide, clobetasol propionate, and dexamethasone;immunosuppressive and/or immunomodulatory agents such asanti-proliferative, cell-cycle inhibitors (e.g., paclitaxel,cytochalasin D, infiximab), taxol, actinomycin, mitomycin, thospromoteVEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast, actinomycin,everolimus, methothrexate, mycophenolic acid, angiopeptin, vincristing,mitomycine, statins, C MYC antisense, sirolimus (and analogs),RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolylhydroxylase inhibitors, PPARγ ligands (for example troglitazone,rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors,probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelininhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins(for example, Cerivasttin), E. coli heat-labile enterotoxin, andadvanced coatings; anti-infective agents, such as anthelmintics(mebendazole); antibiotics such as aminoclycosides (gentamicin,neomycin, tobramycin), antifungal antibiotics (amphotericin b,fluconazole, griseofulvin, itraconazole, ketoconazole, nystatin,micatin, tolnaftate), cephalosporins (cefaclor, cefazolin, cefotaxime,ceftazidime, ceftriaxone, cefuroxime, cephalexin), beta-lactamantibiotics (cefotetan, meropenem), chloramphenicol, macrolides(azithromycin, clarithromycin, erythromycin), penicillins (penicillin Gsodium salt, amoxicillin, ampicillin, dicloxacillin, nafcillin,piperacillin, ticarcillin), tetracyclines (doxycycline, minocycline,tetracycline), bacitracin; clindamycin; colistimethate sodium; polymyxinb sulfate; vancomycin; antivirals including acyclovir, amantadine,didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine,nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir,valganciclovir, zidovudine; quinolones (ciprofloxacin, levofloxacin);sulfonamides (sulfadiazine, sulfisoxazole); sulfones (dapsone);furazolidone; metronidazole; pentamidine; sulfanilamidum crystallinum;gatifloxacin; and sulfamethoxazole/trimethoprim; interferant scavengers,such as superoxide dismutase (SOD), thioredoxin, glutathione peroxidaseand catalase, anti-oxidants, such as uric acid and vitamin C, ironcompounds, Heme compounds, and some heavy metals; artificial protectivecoating components, such as albumin, fibrin, collagen, endothelialcells, wound closure chemicals, blood products, platelet-rich plasma,growth factors and the like.

In some embodiments, wounding can be suppressed by inclusion of asilicone coating (e.g., silicon-hydrophilic polymer blend) or ahydrophilic shedding layer can be applied to the sensor. While notwishing to be bound by theory, it is believed that a siliconebioprotective coating or shedding layer can promote formation andmaintenance of a fluid pocket around the sensor, to enhance glucose andfluid transport as well as clearance of interferants. A siliconebioprotective coating can create a local environment with enhancedvascular permeability and/or vascularization. Such a coating is believedto speed up the inflammatory response to achieve a substantiallyconsistent wound environment more quickly than without the coating.Furthermore, a silicone bioprotective coating is believed to be able tosubdue the inflammatory response to reduce production of cellularbyproducts that are believed to be electrochemical interferants.

In one embodiment, a silicone bioprotective coating can consist of oneor more layer(s) formed from a composition that, in addition toproviding high oxygen solubility, allows for the transport of glucose orother such water-soluble molecules (for example, drugs). In oneembodiment, these layers comprise a blend of a silicone polymer with ahydrophilic polymer. For additional description, see the sectionentitled “Noise Reduction by silicon/Hydrophilic Polymer BlendMaterials,” and co-pending U.S. patent application Ser. No. 11/404,417,filed Apr. 14, 2006 and entitled “SILICONE BASED MEMBRANES FOR USE INIMPLANTABLE GLUCOSE SENSORS,” co-pending U.S. patent application Ser.No. 11/675,063, filed Feb. 14, 2007 and entitled “ANALYTE SENSOR,” U.S.Patent Publication No. US-2005-0090607-A1, U.S. Patent Publication No.US-2006-0270923-A1, and U.S. Patent Publication No. US-2007-0027370-A1,each of which are incorporated by reference herein in their entirety.

Many of the above disclosed methods and structures for forming a fluidpocket, diluting interferants, reducing noise and the like can be usedin combination to facilitate a desired effect or outcome. For example,in one embodiment, a shedding layer composed of a hydrophilic siliconefilm and a necrosing agent can be applied in combination to at least aportion of the sensor. The silicone film can suppress protein adherenceto the sensor surface while the necrosing agent can devitalize a smallportion of tissue adjacent to the sensor, stimulating formation of afluid pocket around the hydrophilic silicone film. Preferably, theincreased volume of fluid surrounding the sensor dilutes interferantswhile the shedding layer provides a physical separation between thesensor and the surrounding tissue.

In another exemplary embodiment, a mesh sprayed with dexamethasone iswrapped around the exterior of the sensor. The mesh can provide aphysical spacer for a fluid pocket while the dexamethasone inhibitsinflammation. Preferably, fluid can fill the mesh and the dexamethasonecan promote normal tissue metabolism around the sensor by inhibiting aninflux of inflammatory cells. Consequently, glucose and oxygen cantravel freely between the tissue and the sensor through the fluid filledmesh without a buildup of interferants, even during periods of tissuecompression, thereby promoting sensor sensitivity and thereby reducingnoise.

Additional description of increasing fluid bulk, by adapting thesensor's configuration can be found in co-pending U.S. PatentPublication No. US-2006-0229512-A1 and co-pending U.S. patentapplication Ser. No. 11/654,140, filed on Jan. 17, 2007 and entitled“MEMBRANES FOR ANALYTE SENSOR,” both of which are incorporated herein intheir entirety.

Auxiliary Electrode

In some circumstances, non-constant noise can be reduced byincorporating into the sensor system an auxiliary electrode configuredto electrochemically modify (for example, oxidize or reduce)electrochemical interferants to render them substantiallynon-electroactively reactive at the electroactive sensing surface(s) inorder to overcome the effects of interferants on the working electrode.It is known that many electrochemical interferants can be reduced at apotential of from about +0.1V to +1.2V or more; for example,acetaminophen is reduced at a potential of about +0.4 V. It is notedthat one challenge to generating oxygen electrochemically in this way isthat while an auxiliary electrode does produce excess oxygen, theplacement of the auxiliary electrode in proximity to theanalyte-measuring working electrode can cause oxidation of hydrogenperoxide at the auxiliary electrode, resulting in reduced signals at theworking electrode. Accordingly, the sensors of preferred embodimentsplace an auxiliary electrode above the electrode system, or otherelectroactive sensing surface, thereby reducing or eliminating theproblem of inaccurate signals as described above.

Preferably, the auxiliary electrode is located within or adjacent to themembrane system, for example, between the enzyme and other domains,although the auxiliary electrode can be placed anywhere between theelectroactive sensing surface and the outside fluid. The auxiliaryelectrode is formed from known working electrode materials (for example,platinum, palladium, graphite, gold, carbon, conductive polymer, or thelike) and has a voltage setting that produces oxygen (for example, fromabout +0.6 V to +1.2 V or more) and/or that electrochemically modifies(for example, reduces) electrochemical interferants to render themsubstantially non-reactive at the electroactive sensing surface(s) (forexample, from about +0.1 V to +1.2 V or more). The auxiliary electrodecan be a mesh, grid, plurality of spaced wires or conductive polymers,or other configurations designed to allow analytes to penetratetherethrough.

In another aspect of the preferred embodiments, the auxiliary electrodeis configured to electrochemically modify (for example, oxidize orreduce) electrochemical interferants to render them substantiallynon-reactive at the electroactive sensing surface(s). In theseembodiments, which can be in addition to or alternative to theabove-described oxygen-generating embodiments, a polymer coating ischosen to selectively allow interferants (for example, urate, ascorbate,and/or acetaminophen such as described in U.S. Pat. No. 6,579,690 toBonnecaze, et al.) to pass through the coating and electrochemicallyreact with the auxiliary electrode, which effectively pre-oxidizes theinterferants, rendering them substantially non-reactive at the workingelectrode. In one exemplary embodiment, silicone materials can besynthesized to allow the transport of oxygen, acetaminophen and otherinterferants, but not allow the transport of glucose. In someembodiments, the polymer coating material can be chosen with a molecularweight that blocks glucose and allows the transport of oxygen, urate,ascorbate, and acetaminophen. In another exemplary embodiment, siliconematerials can be synthesized to allow the transport of oxygen, glucose,acetaminophen, and other interferants. In some embodiments, the polymercoating material is chosen with a molecular weight that allows thetransport of oxygen, glucose, urate, ascorbate, and acetaminophen. Thevoltage setting necessary to react with interfering species depends onthe target electrochemical interferants, for example, from about +0.1 Vto about +1.2 V. In some embodiments, wherein the auxiliary electrode isset at a potential of from about +0.6 to about +1.2 V, bothoxygen-generation and electrochemical interferant modification can beachieved. In some embodiments, wherein the auxiliary electrode is set ata potential below about +0.6 V, the auxiliary electrode will functionmainly to electrochemically modify interferants, for example. Additionaldescription can be found in U.S. Pat. No. 7,074,307, to Simpson, whichis incorporated herein by reference.

Interferent Affinity Domain

Incorporating an affinity domain (e.g., affinity for an interferent)into the membrane can reduce non-constant noise by preventing thepassage of an interferent (to which the affinity domain has affinity)therethrough. For example, an affinity domain can be configured topreferentially bind acetaminophen and thereby remove the acetaminophenfrom the cellular milieu surrounding an implanted sensor. In preferredembodiments, the affinity domain can be configured to include anaffinity for numerous other interferants. For example, optical glucosesensors suffer from interference from species such as triglyceride,albumin, and gamma globulin. In general, the effects of any knowninterferants on sensor signals may be reduced using the conceptsdescribed herein.

FIG. 16 is a graph of interferant concentration (relative) versus time(relative), which illustrates the rise and fall of a transientinterferant concentration exposed to a sensor in a host's body. Forexample, when acetaminophen is taken orally, the systemic concentrationrises quickly and then decreases roughly logarithmically as the speciesis cleared by the system, such as illustrated in FIG. 16, line 1602. Itis noted that medication such as acetaminophen is typically takentransiently (e.g., rather than continually) and therefore producestransient, non-glucose related signal artifacts on a glucose-measuringdevice. Because an elevated acetaminophen concentration is a transientevent in the host, moderating acetaminophen concentration is generallyonly required for discrete periods of time.

According to the preferred embodiments, the affinity domain has an“affinity” for the interferant to be blocked, and therefore sorbs thatinterferant; by sorbing the interferant into the membrane system, theeffects on the resulting signal are reduced. Consequently, the localconcentration of interferant presented to the electrochemically reactivesurface of the sensor is moderated as illustrated in FIG. 16, line 1604.

While not wishing to be bound to theory, it is believed that the areaunder both curves is substantially equal, however the localconcentration of interfering species at the sensor with the affinitydomain of the preferred embodiments is sufficiently lowered over time(e.g., line 1604), as compared to a membrane system without the affinitydomain (e.g., line 1602). In other words, the affinity domain of thepreferred embodiments slows the diffusion of the interfering species onthe signal, such that the signal deviation due to the interferant isbelow a level that may substantially interfere with sensor accuracy.

The preferred embodiments provide a membrane system, particularly foruse on an electrochemical sensor, wherein the membrane system includesan affinity domain. The affinity domain can be layer, surface, region,and/or portion of the membrane system and manufactured using a varietyof methods. In general, the affinity domain is formed using sorbentswith an affinity for the target interferant(s). Sorbents include anysubstance (e.g., molecule, particle, coating, or the like) that has astronger affinity for a particular molecule or compound (e.g.,interfering species) than another (e.g., measured analyte or substance).The sorbents of the preferred embodiments provide for the retention ofan interfering species, such that the interfering species will be atleast temporarily immobilized, and will take a longer time to passthrough the affinity domain.

In some embodiments, the sorbents are polymeric adsorbents, such aschromatography-packing materials. The chromatography-packing materialscan be selected, modified, or otherwise adapted to possess an affinityfor a target interferant, for example, phenol-containing species. Someexamples of chromatography-packing materials include Optipore L-493 (DowChemical Company, Providence, R.I.), SP-850 (Mitsubishi ChemicalAmerica, White Plains, N.Y.), Amberlite XAD-4 (Rohm and Haas,Philadelphia, Pa.), and LC-18 (Supleco, Bellefonte, Pa.).

In some embodiments, fused silica, Amberlite XAD-2, Amberlite IRC-50,Discovery DPA-6s, C-6 Bulk Phenyl, and other affinity-based packings oradsorbents synthesized from fused silica and/or TEOS with differentphenyl derivatized silanes, can be used as the sorbents. In someembodiments, the sorbents are formed from carbon-based solids.

In some embodiments, sorbents are coated onto an inert support material,such as treated diatomaceous earth or other silica based materials (forexample, solid silica support particles can have an organic coatingbonded to their surface, wherein the bonding is produced by reacting ahalogen substituted organosilane with the surface —OH groups present onthe silica support). Generally, these coatings are non-polar in natureand therefore retention of the interfering species is produced bydispersion forces, making them useful for separation of organiccompounds based on slight differences in their backbone or side chainconfiguration.

In some embodiments, the affinity domain can be manufactured usingmolecular imaging technology. In this embodiment, a sorbent is selectedor prepared that is useful for binding a pre-determined interferant onthe surface of a material by complementary functional group interaction.For example, a cross-linked styrene divinyl benzene material can beprepared that is imprinted with acetaminophen. U.S. Pat. Nos. 5,453,199and 5,872,198, both of which are incorporated by reference herein intheir entirety, describe molecular imaging technology that can be usedfor imprinting acetaminophen or other interferants on the surface of amaterial. Complementary functional group interaction provides aselective, reversible association between the interferant and thematerial surface. Such methods for making binding surfaces are referredto hereinafter as “molecular imaging” methods and form surfaces referredto hereinafter as “imaged surfaces.”

Molecular imaging provides a high surface area chromatography matrixmaterial with molecular-specific sorbents. The imaged surfaces bind withinterferants by covalently adhering, in a way that is geometricallycontrolled at least in the direction parallel, and preferably also in adirection normal to an underlying surface plane, a plurality of chargedgroups, hydrophobic groups, and various combinations thereof, to form amirror image of groups complementary to them on a molecular surface of atarget molecule, for example acetaminophen. These groups are preferablyspaced about a hydrophilic undersurface rich in hydrogen containinggroups and electronegative atoms such as oxygen, nitrogen, phosphorus,or sulfur that take part in formation of hydrogen bonds.

In some embodiments, a silica-like sol-gel material is imaged similarlyto that described above with reference to molecular imaging. U.S. Pat.No. 6,057,377, which is incorporated herein by reference in itsentirety, describes a method for molecularly imprinting the surface of asol-gel material, by forming a solution including a sol-gel material, asolvent, an imprinting molecule, and a functionalizing siloxane monomerof the form Si(OR)₃-n X_(n), wherein n is an integer between zero andthree and X is a functional group capable of reacting or associatingwith the imprinting molecule. In some embodiments, the phenyl silane isphenyldimethylpropytrimethoxysilane,N-phenylaminopropyltrimethoxysilane, phenyldiethoxysilane, orphenyltriethoxysilane, for example.

The resulting sol-gel structure would include a three dimensionalmaterial imprinted with acetaminophen or other interferant. In thisembodiment, the solvent is evaporated, and the imprinting moleculeremoved to form the molecularly imprinted sol-gel material. The removalof the imprinting molecule creates a pocket, which has the correctgeometry and hydrogen binding to bind the interfering species as itpasses through the structure. This sol-gel structure can then be groundusing a mortar-pestal, or the like, and added to the membrane system asthe affinity domain.

The use of sol-gel materials advantageously allow the material porosity,pore size, density, surface area, hardness, electrostatic charge,polarity, optical density, and surface hydrophobicity to be tailored tosuit the affinity domain useful in the preferred embodiments.

Additional description can be found in U.S. Patent Publication NumberUS-2005-0176136-A1, which is incorporated herein by reference.

Interferent Blocking Compounds

In some embodiments, constant and/or non-constant noise can be decreasedby including one or more layers comprising an interferent-blockingcompound in an interference domain of the membrane system. A variety ofinterferent-blocking compounds can be used, such as but not limited tosulfonated polyether sulfone, polyamino-phenol or polypyrrole. In oneembodiment, the membrane system includes 3-amino-phenol, which allowsthe diffusion of H₂O₂ while blocking the transport of acetaminophen.Interferent-blocking compounds can be applied to the electrodes usingany method know in the art, such as but not limited to dipping,spraying, electro-polymerization, spin coating and the like, as arediscussed elsewhere herein. In one exemplary embodiment, the sensor is aglucose sensor comprising two working electrodes, wherein a solution of3-amino-phenol is sprayed onto the working electrodes and dried prior tothe application of the membrane enzyme domain. In a further embodiment,the sensor includes additional membrane layers. Additional methods anddevices can be found in U.S. Pat. No. 7,120,483, to Russell, which isincorporated herein by reference in its entirety.

Interferent Scavenging

In some embodiments, one or more layers of the membrane system includean interferent scavenger. Depending upon the nature of the interferent,the interferent scavenger can be incorporated into a membrane layereither more distal or proximal to the electroactive surfaces than theenzyme domain; in some embodiments, the scavenger can be incorporatedinto the membrane's enzyme layer. For example, some interferents areionic and bind to ionic interferents. Accordingly, incorporatinginterferent-scavenging ionic components, such as Nafion®, into one ormore layers of the membrane system can substantially block and/or slowdiffusion of an interferent having the same charge as the ioniccomponent through the membrane system, in some embodiments. Thus, lessinterferent reaches the electroactive surfaces and noise is reduced.

An interferent-scavenging enzyme can be incorporated into one or morelayers of the membrane system. Useful enzymes include but are notlimited to peroxidases and/or oxidases. In general, a peroxidasecatalyzes the reduction of a compound using H₂O₂. Exemplary peroxidasesinclude horseradish peroxidase, glutathione peroxidase, cytochrome Cperoxidase, myeloperoxidase, and the like. Horseradish peroxidase is apreferred peroxidase because interferents present in biological fluids,such as ascorbate, urate, acetaminophen, bilirubin and cysteine, arerapidly oxidized by hydrogen peroxide in the presence of horseradishperoxidase. In general, an oxidase catalyzes the oxidation/reduction ofa compound using molecular O₂. Exemplary oxidases include glucoseoxidase, monoamine oxidase, cytochrome P450 oxidase, NADPH oxidase,cytochrome C oxidase, Xanthine oxidase, L-gulonolactone oxidase, lactateoxidase, lysyl oxidase and the like. In some embodiments, the peroxidasecan be crosslinked to one or more membrane domains using known proteincross-linking techniques, such as but not limited to glutaraldehydecross-linking, NaIO₄, oxidation of enzyme oligosaccharide groupsfollowed by coupling to the matrix. Some useful coupling methods aredescribed in U.S. Pat. Nos. 5,262,305 and 5,356,786, incorporated hereinby reference.

In one exemplary embodiment, a peroxidase is incorporated into a distalmembrane domain (e.g., above the enzyme domain) to remove H₂O₂ derivedfrom external sources (e.g., from macrophages during wound healing). Inone exemplary embodiment, the resistance domain 30 compriseshorseradish-peroxidase. In a preferred embodiment, the sensor is aglucose sensor comprising two working electrodes and a resistance domaincomprising horseradish peroxidase. Additional scavenging techniques aredescribed in U.S. Pat. No. 5,356,786 to Heller, U.S. Pat. No. 6,284,478to Heller and U.S. Pat. No. 7,003,341 to Say, which are incorporatedherein by reference in their entirety.

Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system of the preferredembodiments are described in U.S. Patent Publication No.US-2005-0115832-A1, U.S. Patent Publication No. US-2005-0176136-A1, U.S.Patent Publication No. US-2005-0161346-A1, and U.S. Patent PublicationNo. US-2005-0143635-A1, each of which are incorporated herein byreference. In some alternative embodiments, a distinct interferencedomain is not included.

Enzyme Domain

In preferred embodiments, the membrane system further includes an enzymedomain 28 disposed more distally from the electroactive surfaces thanthe interference domain; however other configurations can be desirable(FIGS. 2A-2B). In the preferred embodiments, the enzyme domain providesan enzyme to catalyze the reaction of the analyte and its co-reactant,as described in more detail below. In the preferred embodiments of aglucose sensor, the enzyme domain includes glucose oxidase; howeverother oxidases, for example, galactose oxidase or uricase oxidase, canalso be used.

For an enzyme-based electrochemical glucose sensor to perform well, thesensor's response is preferably limited by neither enzyme activity norco-reactant concentration. Because enzymes, including glucose oxidase(GOx), are subject to deactivation as a function of time even in ambientconditions, this behavior is compensated for in forming the enzymedomain. Preferably, the enzyme domain is constructed of aqueousdispersions of colloidal polyurethane polymers including the enzyme.However, in alternative embodiments the enzyme domain is constructedfrom an oxygen enhancing material, for example, silicone, orfluorocarbon, in order to provide a supply of excess oxygen duringtransient ischemia. Preferably, the enzyme is immobilized within thedomain. See, e.g., U.S. Patent Publication No. US-2005-0054909-A1.

In preferred embodiments, the enzyme domain is deposited onto theinterference domain for a domain thickness of from about 0.05 micron orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. However in some embodiments,the enzyme domain can be deposited directly onto the electroactivesurfaces. Preferably, the enzyme domain is deposited by spray or dipcoating. In one embodiment of needle-type (transcutaneous) sensor suchas described herein, the enzyme domain is formed by dip coating theinterference domain coated sensor into an enzyme domain solution andcuring the domain for from about 15 to about 30 minutes at a temperatureof from about 40° C. to about 55° C. (and can be accomplished undervacuum (e.g., 20 to 30 mmHg)). In embodiments wherein dip coating isused to deposit the enzyme domain at room temperature, a preferredinsertion rate of from about 0.25 inch per minute to about 3 inches perminute, with a preferred dwell time of from about 0.5 minutes to about 2minutes, and a preferred withdrawal rate of from about 0.25 inch perminute to about 2 inches per minute provides a functional coating.However, values outside of those set forth above can be acceptable oreven desirable in certain embodiments, for example, depending uponviscosity and surface tension, as is appreciated by one skilled in theart. In one embodiment, the enzyme domain is formed by dip coating twotimes (namely, forming two layers) in an enzyme domain solution andcuring at 50° C. under vacuum for 20 minutes. However, in someembodiments, the enzyme domain can be formed by dip coating and/or spraycoating one or more layers at a predetermined concentration of thecoating solution, insertion rate, dwell time, withdrawal rate, and/ordesired thickness.

Resistance Domain

In preferred embodiments, the membrane system includes a resistancedomain 30 disposed more distal from the electroactive surfaces than theenzyme domain (FIGS. 2A-2B). Although the following description isdirected to a resistance domain for a glucose sensor, the resistancedomain can be modified for other analytes and co-reactants as well.

There exists a molar excess of glucose relative to the amount of oxygenin blood; that is, for every free oxygen molecule in extracellularfluid, there are typically more than 100 glucose molecules present (seeUpdike et al., Diabetes Care 5:207-21 (1982)). However, an immobilizedenzyme-based glucose sensor employing oxygen as co-reactant ispreferably supplied with oxygen in non-rate-limiting excess in order forthe sensor to respond linearly to changes in glucose concentration,while not responding to changes in oxygen concentration. Specifically,when a glucose-monitoring reaction is oxygen limited, linearity is notachieved above minimal concentrations of glucose. Without asemipermeable membrane situated over the enzyme domain to control theflux of glucose and oxygen, a linear response to glucose levels can beobtained only for glucose concentrations of up to about 40 mg/dL.However, in a clinical setting, a linear response to glucose levels isdesirable up to at least about 400 mg/dL.

The resistance domain includes a semipermeable membrane that controlsthe flux of oxygen and glucose to the underlying enzyme domain,preferably rendering oxygen in a non-rate-limiting excess. As a result,the upper limit of linearity of glucose measurement is extended to amuch higher value than that which is achieved without the resistancedomain. In one embodiment, the resistance domain exhibits an oxygen toglucose permeability ratio of from about 50:1 or less to about 400:1 ormore, preferably about 200:1. As a result, one-dimensional reactantdiffusion is adequate to provide excess oxygen at all reasonable glucoseand oxygen concentrations found in the subcutaneous matrix (See Rhodeset al., Anal. Chem., 66:1520-1529 (1994)).

In alternative embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubilitydomain (for example, a silicone or fluorocarbon-based material ordomain) to enhance the supply/transport of oxygen to the enzyme domain.If more oxygen is supplied to the enzyme, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.In alternative embodiments, the resistance domain is formed from asilicone composition, such as is described in U.S. Patent PublicationNo. US-2005-0090607-A1, which is incorporated by reference herein.

In a preferred embodiment, the resistance domain includes a polyurethanemembrane with both hydrophilic and hydrophobic regions to control thediffusion of glucose and oxygen to an analyte sensor, the membrane beingfabricated easily and reproducibly from commercially availablematerials. A suitable hydrophobic polymer component is a polyurethane,or polyetherurethaneurea. Polyurethane is a polymer produced by thecondensation reaction of a diisocyanate and a difunctionalhydroxyl-containing material. A polyurethaneurea is a polymer producedby the condensation reaction of a diisocyanate and a difunctionalamine-containing material. Preferred diisocyanates include aliphaticdiisocyanates containing from about 4 to about 8 methylene units.Diisocyanates containing cycloaliphatic moieties can also be useful inthe preparation of the polymer and copolymer components of the membranesof preferred embodiments. The material that forms the basis of thehydrophobic matrix of the resistance domain can be any of those known inthe art as appropriate for use as membranes in sensor devices and ashaving sufficient permeability to allow relevant compounds to passthrough it, for example, to allow an oxygen molecule to pass through themembrane from the sample under examination in order to reach the activeenzyme or electrochemical electrodes. Examples of materials which can beused to make non-polyurethane type membranes include vinyl polymers,polyethers, polyesters, polyamides, inorganic polymers such aspolysiloxanes and polycarbosiloxanes, natural polymers such ascellulosic and protein based materials, and mixtures or combinationsthereof.

In a preferred embodiment, the hydrophilic polymer component ispolyethylene oxide. For example, one useful hydrophobic-hydrophiliccopolymer component is a polyurethane polymer that includes about 20%hydrophilic polyethylene oxide. The polyethylene oxide portions of thecopolymer are thermodynamically driven to separate from the hydrophobicportions of the copolymer and the hydrophobic polymer component. The 20%polyethylene oxide-based soft segment portion of the copolymer used toform the final blend affects the water pick-up and subsequent glucosepermeability of the membrane.

In some embodiments, the resistance domain is formed from a siliconepolymer modified to allow analyte (e.g., glucose) transport.

In some embodiments, the resistance domain is formed from a siliconepolymer/hydrophobic-hydrophilic polymer blend. In one embodiment, Thehydrophobic-hydrophilic polymer for use in the blend may be any suitablehydrophobic-hydrophilic polymer, including but not limited to componentssuch as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein byreference). In one embodiment, the hydrophobic-hydrophilic polymer is acopolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO).Suitable such polymers include, but are not limited to, PEO-PPO diblockcopolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblockcopolymers, alternating block copolymers of PEO-PPO, random copolymersof ethylene oxide and propylene oxide, and blends thereof. In someembodiments, the copolymers may be optionally substituted with hydroxysubstituents. Commercially available examples of PEO and PPO copolymersinclude the PLURONIC® brand of polymers available from BASF®. In oneembodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers includePPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Othersuitable commercial polymers include, but are not limited to,SYNPERONICS® products available from UNIQEMA®.

In preferred embodiments, the resistance domain is deposited onto theenzyme domain to yield a domain thickness of from about 0.05 microns orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. Preferably, the resistancedomain is deposited onto the enzyme domain by vapor deposition, spraycoating, or dip coating. In one preferred embodiment, spray coating isthe preferred deposition technique. The spraying process atomizes andmists the solution, and therefore most or all of the solvent isevaporated prior to the coating material settling on the underlyingdomain, thereby minimizing contact of the solvent with the enzyme.

In a preferred embodiment, the resistance domain is deposited on theenzyme domain by spray coating a solution of from about 1 wt. % to about5 wt. % polymer and from about 95 wt. % to about 99 wt. % solvent. Inspraying a solution of resistance domain material, including a solvent,onto the enzyme domain, it is desirable to mitigate or substantiallyreduce any contact with enzyme of any solvent in the spray solution thatcan deactivate the underlying enzyme of the enzyme domain.Tetrahydrofuran (THF) is one solvent that minimally or negligiblyaffects the enzyme of the enzyme domain upon spraying. Other solventscan also be suitable for use, as is appreciated by one skilled in theart.

Preferably, each exemplary sensor design (e.g., FIGS. 1A, 2A, and 7Athrough 9B) includes electronic connections, for example, one or moreelectrical contacts configured to provide secure electrical contactbetween the sensor and associated electronics. In some embodiments, theelectrodes and membrane systems of the preferred embodiments arecoaxially formed, namely, the electrodes and/or membrane system allshare the same central axis. While not wishing to be bound by theory, itis believed that a coaxial design of the sensor enables a symmetricaldesign without a preferred bend radius. Namely, in contrast to prior artsensors comprising a substantially planar configuration that can sufferfrom regular bending about the plane of the sensor, the coaxial designof the preferred embodiments do not have a preferred bend radius andtherefore are not subject to regular bending about a particular plane(which can cause fatigue failures and the like). However, non-coaxialsensors can be implemented with the sensor system of the preferredembodiments.

In addition to the above-described advantages, the coaxial sensor designof the preferred embodiments enables the diameter of the connecting endof the sensor (proximal portion) to be substantially the same as that ofthe sensing end (distal portion) such that a needle is able to insertthe sensor into the host and subsequently slide back over the sensor andrelease the sensor from the needle, without slots or other complexmulti-component designs, as described in detail in U.S. PatentPublication No. US-2006-0063142-A1 and U.S. application Ser. No.11/360,250 filed Feb. 22, 2006 and entitled “ANALYTE SENSOR,” which areincorporated in their entirety herein by reference.

Exemplary Continuous Sensor Configurations

In some embodiments, the sensor is an enzyme-based electrochemicalsensor, wherein the glucose-measuring working electrode 16 (e.g., FIGS.1A-1B) measures the hydrogen peroxide produced by the enzyme catalyzedreaction of glucose being detected and creates a measurable electroniccurrent (for example, detection of glucose utilizing glucose oxidaseproduces hydrogen peroxide (H₂O₂) as a by product, H₂O₂ reacts with thesurface of the working electrode producing two protons (2H⁺), twoelectrons (2e⁻) and one molecule of oxygen (O₂) which produces theelectronic current being detected, see FIG. 10), such as described inmore detail elsewhere herein and as is appreciated by one skilled in theart. Preferably, one or more potentiostat is employed to monitor theelectrochemical reaction at the electroactive surface of the workingelectrode(s). The potentiostat applies a constant potential to theworking electrode and its associated reference electrode to determinethe current produced at the working electrode. The current that isproduced at the working electrode (and flows through the circuitry tothe counter electrode) is substantially proportional to the amount ofH₂O₂ that diffuses to the working electrodes. The output signal istypically a raw data stream that is used to provide a useful value ofthe measured analyte concentration in a host to the patient or doctor,for example.

Some alternative analyte sensors that can benefit from the systems andmethods of the preferred embodiments include U.S. Pat. No. 5,711,861 toWard et al., U.S. Pat. No. 6,642,015 to Vachon et al., U.S. Pat. No.6,654,625 to Say et al., U.S. Pat. No. 6,565,509 to Say et al., U.S.Pat. No. 6,514,718 to Heller, U.S. Pat. No. 6,465,066 to Essenpreis etal., U.S. Pat. No. 6,214,185 to Offenbacher et al., U.S. Pat. No.5,310,469 to Cunningham et al., and U.S. Pat. No. 5,683,562 to Shafferet al., U.S. Pat. No. 6,579,690 to Bonnecaze et al., U.S. Pat. No.6,484,046 to Say et al., U.S. Pat. No. 6,512,939 to Colvin et al., U.S.Pat. No. 6,424,847 to Mastrototaro et al., U.S. Pat. No. 6,424,847 toMastrototaro et al, for example. All of the above patents areincorporated in their entirety herein by reference and are not inclusiveof all applicable analyte sensors; in general, it should be understoodthat the disclosed embodiments are applicable to a variety of analytesensor configurations.

Although some exemplary glucose sensor configurations are described indetail below, it should be understood that the systems and methodsdescribed herein can be applied to any device capable of continually orcontinuously detecting a concentration of analyte of interest andproviding an output signal that represents the concentration of thatanalyte, for example oxygen, lactose, hormones, cholesterol,medicaments, viruses, or the like.

FIG. 1A is a perspective view of an analyte sensor, including animplantable body with a sensing region including a membrane systemdisposed thereon. In the illustrated embodiment, the analyte sensor 10 aincludes a body 12 and a sensing region 14 including membrane andelectrode systems configured to measure the analyte. In this embodiment,the sensor 10 a is preferably wholly implanted into the subcutaneoustissue of a host, such as described in U.S. Patent Publication No.US-2006-0015020-A1; U.S. Patent Publication No. US-2005-0245799-A1; U.S.Patent Publication No. US-2005-0192557-A1; U.S. Patent Publication No.US-2004-0199059-A1; U.S. Patent Publication No. US-2005-0027463-A1; andU.S. Pat. No. 6,001,067, each of which are incorporated herein byreference in their entirety.

The body 12 of the sensor 10 a can be formed from a variety ofmaterials, including metals, ceramics, plastics, or composites thereof.In one embodiment, the sensor is formed from thermoset molded around thesensor electronics. U.S. Patent Publication No. US-2004-0199059-A1discloses suitable configurations for the body, and is incorporated byreference in its entirety.

In some embodiments, the sensing region 14 includes a glucose-measuringworking electrode 16, an optional auxiliary working electrode 18, areference electrode 20, and a counter electrode 24. Generally, thesensing region 14 includes means to measure two different signals, 1) afirst signal associated with glucose and non-glucose relatedelectroactive compounds having a first oxidation potential, wherein thefirst signal is measured at the glucose-measuring working electrodedisposed beneath an active enzymatic portion of a membrane system, and2) a second signal associated with the baseline and/or sensitivity ofthe glucose sensor. In some embodiments, wherein the second signalmeasures sensitivity, the signal is associated with at least onenon-glucose constant data point, for example, wherein the auxiliaryworking electrode 18 is configured to measure oxygen. In someembodiments, wherein the second signal measures baseline, the signal isassociated with non-glucose related electroactive compounds having thefirst oxidation potential, wherein the second signal is measured at anauxiliary working electrode 18 and is disposed beneath a non-enzymaticportion of the membrane system, such as described in more detailelsewhere herein.

Preferably, a membrane system (see FIG. 2A) is deposited over theelectroactive surfaces of the sensor 10 a and includes a plurality ofdomains or layers, such as described in more detail below, withreference to FIGS. 2A and 2B. In general, the membrane system may bedisposed over (deposited on) the electroactive surfaces using methodsappreciated by one skilled in the art. See U.S. Patent Publication No.US-2006-0015020-A1.

The sensing region 14 comprises electroactive surfaces, which are incontact with an electrolyte phase (not shown), which is a free-flowingfluid phase disposed between the membrane system 22 and theelectroactive surfaces. In this embodiment, the counter electrode isprovided to balance the current generated by the species being measuredat the working electrode. In the case of glucose oxidase based analytesensors, the species being measured at the working electrode is H₂O₂.Glucose oxidase catalyzes the conversion of oxygen and glucose tohydrogen peroxide and gluconate according to the following reaction:Glucose+O₂→Gluconate+H₂O₂

The change in H₂O₂ can be monitored to determine glucose concentrationbecause for each glucose molecule metabolized, there is a proportionalchange in the product H₂O₂ (see FIG. 10). Oxidation of H₂O₂ by theworking electrode is balanced by reduction of ambient oxygen, enzymegenerated H₂O₂, or other reducible species at the counter electrode. TheH₂O₂ produced from the glucose oxidase reaction further reacts at thesurface of the working electrode and produces two protons (2H⁺), twoelectrons (2e⁻), and one oxygen molecule (O₂). Preferably, one or morepotentiostats are employed to monitor the electrochemical reaction atthe electroactive surface of the working electrode(s). The potentiostatapplies a constant potential to the working electrode and its associatedreference electrode to determine the current produced at the workingelectrode. The current that is produced at the working electrode (andflows through the circuitry to the counter electrode) is substantiallyproportional to the amount of H₂O₂ that diffuses to the workingelectrodes. The output signal is typically a raw data stream that isused to provide a useful value of the measured analyte concentration ina host to the patient or doctor, for example.

FIG. 1B is a schematic view of an alternative exemplary embodiment of acontinuous analyte sensor 10 b, also referred to as an in-dwelling ortranscutaneous analyte sensor in some circumstances, particularlyillustrating the in vivo portion of the sensor. In this embodiment, thein vivo portion of the sensor 10 b is the portion adapted for insertionunder the host's skin, in a host's blood stream, or other biologicalsample, while an ex vivo portion of the sensor (not shown) is theportion that remains above the host's skin after sensor insertion andoperably connects to an electronics unit. In the illustrated embodiment,the analyte sensor 10 b is coaxial and includes three electrodes: aglucose-measuring working electrode 16, an optional auxiliary workingelectrode 18, and at least one additional electrode 20, which mayfunction as a counter and/or reference electrode, hereinafter referredto as the reference electrode 20. Generally, the sensor 10 b may includethe ability to measure two different signals, 1) a first signalassociated with glucose and non-glucose related electroactive compoundshaving a first oxidation potential, wherein the first signal is measuredat the glucose-measuring working electrode disposed beneath an activeenzymatic portion of a membrane system, and 2) a second signalassociated with the baseline and/or sensitivity of the glucose sensor,such as described in more detail above with reference to FIG. 1A.

One skilled in the art appreciates that the analyte sensor of FIG. 1Bcan have a variety of configurations. In one exemplary embodiment, thesensor is generally configured of a first working electrode, a secondworking electrode, and a reference electrode. In one exemplaryconfiguration, the first working electrode 16 is a central metal wire orplated non-conductive rod/filament/fiber and the second working andreference electrodes (20 and 18, respectively or 18 and 20,respectively) are coiled around the first working electrode 16. Inanother exemplary configuration, the first working electrode is acentral wire, as depicted in FIG. 1B, the second working electrode iscoiled around the first working electrode, and the reference electrodeis disposed remotely from the sensor, as described herein. In anotherexemplary configuration, the first and second working electrodes (20 and18) are coiled around a supporting rod 16 of insulating material. Thereference electrode (not shown) can be disposed remotely from thesensor, as described herein, or disposed on the non-conductivesupporting rod 16. In still another exemplary configuration, the firstand second working electrodes (20 and 18) are coiled around a referenceelectrode 16 (not to scale).

Preferably, each electrode is formed from a fine wire, with a diameterin the range of 0.001 to 0.010 inches, for example, and may be formedfrom plated wire or bulk material, however the electrodes may bedeposited on a substrate or other known configurations as is appreciatedby one skilled in the art.

In one embodiment, the glucose-measuring working electrode 16 comprisesa wire formed from a conductive material, such as platinum, palladium,graphite, gold, carbon, conductive polymer, or the like. Alternatively,the glucose-measuring working electrode 16 can be formed of anon-conductive fiber or rod that is plated with a conductive material.The glucose-measuring working electrode 16 is configured and arranged tomeasure the concentration of glucose. The glucose-measuring workingelectrode 16 is covered with an insulating material, for example anon-conductive polymer. Dip-coating, spray-coating, or other coating ordeposition techniques can be used to deposit the insulating material onthe working electrode, for example. In one preferred embodiment, theinsulating material comprises Parylene, which can be an advantageousconformal coating for its strength, lubricity, and electrical insulationproperties, however, a variety of other insulating materials can beused, for example, fluorinated polymers, polyethyleneterephthalate,polyurethane, polyimide, or the like.

In this embodiment, the auxiliary working electrode 18 comprises a wireformed from a conductive material, such as described with reference tothe glucose-measuring working electrode 16 above. Preferably, thereference electrode 20, which may function as a reference electrodealone, or as a dual reference and counter electrode, is formed fromsilver, Silver/Silver chloride, or the like.

Preferably, the electrodes are juxtapositioned and/or twisted with oraround each other; however other configurations are also possible. Inone example, the auxiliary working electrode 18 and reference electrode20 may be helically wound around the glucose-measuring working electrode16 as illustrated in FIG. 1B. Alternatively, the auxiliary workingelectrode 18 and reference electrode 20 may be formed as a double helixaround a length of the glucose-measuring working electrode 16. In someembodiments, the working electrode, auxiliary working electrode andreference electrodes may be formed as a triple helix. The assembly ofwires may then be optionally coated together with an insulatingmaterial, similar to that described above, in order to provide aninsulating attachment. Some portion of the coated assembly structure isthen stripped, for example using an excimer laser, chemical etching, orthe like, to expose the necessary electroactive surfaces. In somealternative embodiments, additional electrodes may be included withinthe assembly, for example, a three-electrode system (including separatereference and counter electrodes) as is appreciated by one skilled inthe art.

FIGS. 2A and 2B are schematic views membrane systems in some embodimentsthat may be disposed over the electroactive surfaces of an analytesensors of FIGS. 1A and 1B, respectively, wherein the membrane systemincludes one or more of the following domains: a resistance domain 30,an enzyme domain 28, an interference domain 26, and an electrolytedomain 24, such as described in more detail below. However, it isunderstood that the membrane system 22 can be modified for use in othersensors, by including only one or more of the domains, additionaldomains not recited above, or for other sensor configurations. Forexample, the interference domain can be removed when other methods forremoving interferants are utilized, such as an auxiliary electrode formeasuring and subtracting out signal due to interferants. As anotherexample, an “oxygen antenna domain” composed of a material that hashigher oxygen solubility than aqueous media so that it concentratesoxygen from the biological fluid surrounding the biointerface membranecan be added. The oxygen antenna domain can then act as an oxygen sourceduring times of minimal oxygen availability and has the capacity toprovide on demand a higher rate of oxygen delivery to facilitate oxygentransport to the membrane. This enhances function in the enzyme reactiondomain and at the counter electrode surface when glucose conversion tohydrogen peroxide in the enzyme domain consumes oxygen from thesurrounding domains. Thus, this ability of the oxygen antenna domain toapply a higher flux of oxygen to critical domains when needed improvesoverall sensor function.

In some embodiments, the membrane system generally provides one or moreof the following functions: 1) protection of the exposed electrodesurface from the biological environment, 2) diffusion resistance(limitation) of the analyte, 3) a catalyst for enabling an enzymaticreaction, 4) optionally limitation or blocking of interfering species,and 5) hydrophilicity at the electrochemically reactive surfaces of thesensor interface, such as described in U.S. Patent Publication No.US-2005-0245799-A1. In some embodiments, the membrane systemadditionally includes a cell disruptive domain, a cell impermeabledomain, and/or an oxygen domain (not shown), such as described in moredetail in U.S. Patent Publication No. US-2005-0245799-A1. However, it isunderstood that a membrane system modified for other sensors, forexample, by including fewer or additional domains is within the scope ofthe preferred embodiments.

One aspect of the preferred embodiments provides for a sensor (fortranscutaneous, wholly implantable, or intravascular short-term orlong-term use) having integrally formed parts, such as but not limitedto a plurality of electrodes, a membrane system and an enzyme. Forexample, the parts may be coaxial, juxtapositioned, helical, bundledand/or twisted, plated and/or deposited thereon, extruded, molded, heldtogether by another component, and the like. In another example, thecomponents of the electrode system are integrally formed, (e.g., withoutadditional support, such as a supporting substrate), such thatsubstantially all parts of the system provide essential functions of thesensor (e.g., the sensing mechanism or “in vivo” portion). In a furtherexample, a first electrode can be integrally formed directly on a secondelectrode (e.g. electrically isolated by an insulator), such as by vapordeposition of a conductive electrode material, screen printing aconductive electrode ink or twisting two electrode wires together in acoiled structure.

Some embodiments provide an analyte sensor that is configured forinsertion into a host and for measuring an analyte in the host, whereinthe sensor includes a first working electrode disposed beneath an activeenzymatic portion of a membrane (e.g., membrane system) on the sensorand a second working electrode disposed beneath an inactive- ornon-enzymatic portion of the membrane on the sensor. In theseembodiments, the first and second working electrodes integrally form atleast a portion of the sensor.

Exemplary Sensor Configurations

FIG. 1B is a schematic view of a sensor in one embodiment. In somepreferred embodiments, the sensor is configured to be integrally formedand coaxial. In this exemplary embodiment, one or more electrodes arehelically wound around a central core, all of which share axis A-A. Thecentral core 16 can be an electrode (e.g., a wire or metal-platedinsulator) or a support made of insulating material. The coiledelectrodes 18, 20 are made of conductive material (e.g., plated wire,metal-plated polymer filaments, bulk metal wires, etc.) that ishelically wound or twisted about the core 16. Generally, at least theworking electrodes are coated with an insulator I of non-conductive ordielectric material.

One skilled in the art will recognize that various electrodecombinations are possible. For example, in one embodiment, the core 16is a first working electrode and can be substantially straight. One ofthe coiled electrodes (18 or 20) is a second working electrode and theremaining coiled electrode is a reference or counter electrode. In afurther embodiment, the reference electrode can be disposed remotelyfrom the sensor, such as on the host's skin or on the exterior of thesensor, for example. Although this exemplary embodiment illustrates anintegrally formed coaxial sensor, one skilled in the art appreciates avariety of alternative configurations. In one exemplary embodiment, thearrangement of electrodes is reversed, wherein the first workingelectrode is helically wound around the second working electrode core16. In another exemplary embodiment, the reference electrode can formthe central core 16 with the first and second working electrodes coiledthere around. In some exemplary embodiments, the sensor can haveadditional working, reference and/or counter electrodes, depending uponthe sensor's purpose. Generally, one or more of the electrode wires arecoated with an insulating material, to prevent direct contact betweenthe electrodes. Generally, a portion of the insulating material can beremoved (e.g., etched, scraped or grit-blasted away) to expose anelectroactive surface of the electrode. An enzyme solution can beapplied to the exposed electroactive surface, as described herein.

The electrodes each have first and second ends. The electrodes can be ofany geometric solid shape, such as but not limited to a cylinder havinga circular or oval cross-section, a rectangle (e.g., extrudedrectangle), a triangle (e.g., extruded triangle), an X-cross section, aY-cross section, flower petal-cross sections, star-cross sections,melt-blown fibers loaded with conductive material (e.g., conductivepolymers) and the like. The first ends (e.g., an in vivo portion, “frontend”) of the electrodes are configured for insertion in the host and thesecond ends (e.g., an ex vivo portion, “back end”) are configured forelectrical connection to sensor electronics. In some embodiments, thesensor includes sensor electronics that collect data from the sensor andprovide the data to the host in various ways. Sensor electronics arediscussed in detail elsewhere herein.

FIGS. 7A1 and 7A2 are schematics of an analyte sensor in anotherembodiment. FIG. 7A1 is a side view and FIG. 7A2 is a side-cutaway view.In some preferred embodiments, the sensor is configured to be integrallyformed and coaxial, with an optional stepped end. In this exemplaryembodiment, the sensor includes a plurality of electrodes E1, E2, E3 toEn, wherein n equals any number of electrode layers. Layers ofinsulating material I (e.g., non-conductive material) separate theelectrode layers. All of the electrode and insulating material layersshare axis A-A. The layers can be applied by any technique known in theart, such as but not limited to spraying, dipping, spraying, etc. Forexample, a bulk metal wire electrode E1 can be dipped into a solution ofinsulating polymer that is vulcanized to form a layer of non-conductive,electrically insulating material I. A second electrode E2 can be plated(e.g., by electroplating or other plating technique used in the art) onthe first insulating layer, followed by application of a secondinsulating layer I applied in the same manner as the first layer.Additional electrode layers (e.g., E3 to En) and insulating layers canbe added to the construct, to create the desired number of electrodesand insulating layers. As an example, multiple sensors can be formedfrom a long wire (with insulating and electrode layers applied) that canbe cut to yield a plurality of sensors of the desired length. After thesensor has been cut to size, it can be polished or otherwise treated toprepare the electrodes for use. In some embodiments, the variouselectrode and/or insulator layers can be applied by dipping, spraying,printing, vapor deposition, plating, spin coating or any other methodknown in the art. Although this exemplary embodiment illustrates anintegrally formed coaxial sensor, one skilled in the art appreciates avariety of alternative configurations. For example, in some embodiments,the sensor can have two, three, four or more electrodes separated byinsulating material I. In another embodiment, the analyte sensor has twoor more electrodes, such as but not limited to a first workingelectrode, an auxiliary working electrode, a reference electrode and/orcounter electrode. FIG. 7B is a schematic view of an integrally formed,coaxial sensor in another embodiment. In this exemplary embodiment, acoiled first electrode E1 is manufactured from an electricallyconductive tube or cylinder, such as but not limited to a silverHypotube. A portion of the Hypotube is trimmed or carved into a helix orcoil 702. A second electrode E2 that is sized to fit (e.g., with minimaltolerance) within the first electrode E1 mates (e.g., slides into) withthe first electrode E1, to form the sensor. In general, the surfaces ofthe electrodes are coated with an insulator, to prevent direct contactbetween the electrodes. As described herein, portion of the insulatorcan be stripped away to expose the electroactive surfaces. Although thisexemplary embodiment illustrates one configuration of a coaxial,integrally formed sensor, one skilled in the art appreciates a varietyof alternative configurations. For example, in some embodiments, thefirst electrode E1 is a reference or auxiliary electrode, and the secondelectrode E2 is a working electrode. However, the first electrode E1 canbe a working electrode and the second electrode E2 can be a reference orauxiliary electrode. In some embodiments, additional electrodes areapplied to the construct (e.g., after E2 is inserted into E1). Oneadvantage of this configuration is that the silver Hypotube can be cutto increase or decrease the flexibility of the sensor. For example, thespiral cut can space the coils farther apart to increase the sensor'sflexibility. Another example of this configuration is that it is easierto construct the sensor in this manner, rather than winding oneelectrode around another (e.g., as is done for the embodiment shown inFIG. 1B).

FIGS. 7C to 7E are schematics of three embodiments of bundled analytesensors. In these embodiments, of the sensors are configured to beintegrally formed sensors, wherein a plurality (E1, E2, E3, to En) ofelectrodes are bundled, coiled or twisted to form a portion of thesensor. In some embodiments, the electrodes can be twisted or helicallycoiled to form a coaxial portion of the sensor, which share the sameaxis. In one embodiment, the first and second working electrodes aretwisted or helically wound together, to form at least a portion of thesensor (e.g., a glucose sensor). For example, the electrodes can betwisted in a double helix. In some embodiments, additional electrodesare provided and twisted, coiled or wound with the first and secondelectrodes to form a larger super helix, such as a triple helix, aquadruple helix, or the like. For example, three wires (E1, E2, and E3)can be twisted to form a triple helix. In still other embodiments, atleast one reference electrode can be disposed remotely from the workingelectrodes, as described elsewhere herein. In some embodiments, the tipof the sensor can be cut at an angle (90° or other angle) to expose theelectrode tips to varying extents, as described herein.

FIG. 7C is a schematic of an exemplary embodiment of a sensor havingthree bundled electrodes E1, E2, and E3. In some preferred embodimentsof the sensor, two or all of the electrodes can be identical.Alternatively, the electrodes can be non-identical. For example, thesensor can have a glucose-sensing electrode, an oxygen-sensing electrodeand a reference electrode. Although this exemplary embodimentillustrates a bundled sensor, one skilled in the art appreciates avariety of alternative sensor configurations. For example, only twoelectrodes can be used or more than three electrodes can be used. Inanother example, holding one end of the bundled wires in a clamp andtwisting the other end of the wires, to form a cable-like structure, cancoil the electrodes together. Such a coiled structure can hold theelectrodes together without additional structure (e.g., bound by a wireor coating). In another example, non-coiled electrodes can be bundledand held together with a wire or fiber coiled there around, or byapplying a coating of insulating material to the electrode bundle. Instill another example, the reference electrode can be disposed remotelyfrom the working electrodes, as described elsewhere herein.

FIG. 7D is a schematic view of a sensor in one embodiment. In somepreferred embodiments, the sensor is designed to be integrally formedand bundled and/or coaxial. In this exemplary embodiment, the sensorincludes seven electrodes, wherein three electrodes of a first type(e.g., 3×E1) and three electrodes of a second type (e.g., 3×E2) arebundled around one electrode of a third type (e.g., E3). Those skilledin the art appreciate a variety of configurations possible with thisembodiment. For example, the different types of electrodes can bealternated or not alternated. For example, in FIG. 7D, the two types ofelectrodes are alternately disposed around E3. However, the two types ofelectrodes can be grouped around the central structure. As describedherein, some or all of the electrodes can be coated with a layer ofinsulating material, to prevent direct contact between the electrodes.The electrodes can be coiled together, as in a cable, or held togetherby a wire or fiber wrapping or a coating of insulating material. Thesensor can be cut, to expose the electroactive surfaces of theelectrodes, or portions of the insulating material coating can bestripped away, as described elsewhere herein. In another example, thesensor can include additional (or fewer) electrodes. In one exemplaryembodiment, the E1 and E2 electrodes are bundled around a non-conductivecore (e.g., instead of electrode E3), such as an insulated fiber. Inanother embodiment, different numbers of E1, E2, and E3 electrodes canbe used (e.g., two E1 electrodes, two E2 electrodes, and three E3electrodes). In another embodiment, additional electrode type can beincluded in the sensor (e.g., an electrode of type E4, E5 or E6, etc.).In still another exemplary embodiment, three glucose-detectingelectrodes (e.g., E1) and three reference electrodes (e.g., E2) arebundled and (optionally) coiled around a central auxiliary workingelectrode (e.g., E3).

FIG. 7E is a schematic of a sensor in another embodiment. In thisexemplary embodiment of an integrally formed sensor, two pairs ofelectrodes (e.g., 2×E1 and 2×E2) are bundled around a core of insulatingmaterial I. Fibers or strands of insulating material I also separate theelectrodes from each other. Although this exemplary embodimentillustrates an integrally formed sensor, one skilled in the artappreciates a variety of alternative configurations. For example, thepair of E1 electrodes can be working electrodes and the pair of E2electrodes can be reference and/or auxiliary electrodes. In oneexemplary embodiment, the E1 electrodes are both glucose-detectingelectrodes, a first E2 electrode is a reference electrode and a secondE2 electrode is an auxiliary electrode. In another exemplary embodiment,one E1 electrode includes active GOx and measures a glucose-relatedsignal; the other E1 electrode lacks active GOx and measures anon-glucose-related signal, and the E2 electrodes are referenceelectrodes. In yet another exemplary embodiment, one E1 electrodedetects glucose and the other E1 electrode detects urea, and both E2electrodes are reference electrodes. One skilled in the art ofelectrochemical sensors will recognized that the size of the variouselectrodes can be varied, depending upon their purpose and the currentand/or electrical potential used. Electrode size and insulating materialsize/shape are not constrained by their depiction of relative size inthe Figures, which are schematic schematics intended for onlyillustrative purposes.

FIG. 7F is a schematic view of a cross-section of an integrally formedsensor in another embodiment. In some preferred embodiments, the sensoris configured to be bifunctional. In this exemplary embodiment, thesensor includes two working electrodes E1/E2 separated by either areference electrode R or an insulating material I. The electrodes E1, E2and optionally the reference electrode R are conductive and support thesensor's shape. In addition, the reference electrode R (or theinsulating material I) can act as a diffusion barrier (D, describedherein) between the working electrodes E1, E2 and support the sensor'sstructure. Although this exemplary embodiment illustrates oneconfiguration of an integrally formed sensor having bifunctionalcomponents, one skilled in the art appreciates a variety of alternativeconfigurations. Namely, FIG. 7F is not to scale and the workingelectrodes E1, E2 can be relatively larger or smaller in scale, withregard to the reference electrode/insulator R/I separating them. Forexample, in one embodiment, the working electrodes E1, E2 are separatedby a reference electrode that has at least 6-times the surface area ofthe working electrodes, combined. While the working electrodes E1, E2and reference electrode/insulator R/I are shown and semi-circles and arectangle, respectively, one skilled in the art recognizes that thesecomponents can take on any geometry know in the art, such as but notlimited to rectangles, cubes, cylinders, cones, and the like.

FIG. 7G is a schematic view of a sensor in yet another embodiment. Insome preferred embodiments, the sensor is configured to be integrallyformed with a diffusion barrier D, as described herein. In thisexemplary embodiment, the working electrodes E1, E2 (or one workingelectrode and one counter electrode) are integrally formed on asubstantially larger reference electrode R or an insulator I thatsubstantially prevents diffusion of analyte or other species from oneworking electrode to another working electrode (e.g., from the enzymaticelectrode (e.g., coated with active enzyme) to the non-enzymaticelectrode (e.g., no enzyme or inactive enzyme)). Although this exemplaryembodiment illustrates an integrally formed sensor having a diffusionbarrier, one skilled in the art appreciates a variety of alternativeconfigurations. For example, in one embodiment, the reference electrodeis designed to include an exposed electroactive surface area that is atleast equal to, greater than, or more than about 2, 3, 4, 5, 6, 7, 8, 9,10 or more times greater than the surface area of the working electrodes(e.g., combined). In other embodiments, the surface of the referenceelectrode is about 6 (e.g., about 6 to 20) or more times greater thanthe working electrodes. In some embodiments, each working electrodedetects a separate analyte (e.g., glucose, oxygen, uric acid, nitrogen,pH, and the like). In other embodiments, one of the working electrodesis a counter electrode. In still another exemplary embodiment, an enzymesolution containing active GOx is applied to the E1 electroactivesurface, while an enzyme solution containing inactive GOx (or no GOx atall) is applied to the E2 electroactive surface. As described herein,this configuration allows the measurement of two signals. Electrode E1measures both a signal related to glucose concentration and a signalthat is not related to glucose concentration. Electrode E2 measures asignal that is not related to glucose concentration. The sensorelectronics, as described herein, can use these data to calculateglucose concentration without signal due to non-glucose-relatedcontributions.

FIG. 7H is a schematic view of a sensor in another embodiment. In somepreferred embodiments, the sensor is configured of a geometric solid(e.g., cylindrical) reference electrode R having two or more workingelectrodes E1, E2 to En disposed within two or more grooves or channelscarved in the sides of the reference electrode R (parallel to the axisof the reference electrode R). The grooves are sized such that theelectrodes E1, E2 can snuggly fit therein. Additionally, the depth ofthe grooves can be configured that the electrode placed therein isexternally exposed to a greater or lesser degree. For example, theopening to the groove may be wider or narrower. In some embodiments, aportion of an electrode protrudes from the groove in which the electrodehas been disposed. In some embodiments, an insulator (e.g., I) takes theplace of a reference electrode (which can be disposed elsewhere, suchremotely as described in more detail elsewhere herein). The referenceelectrode/insulator R/I can take any geometric structure known in theart, such as but not limited to cylinders, rectangles, cones, and thelike. Similarly, the relative sizes of the working electrodes E1, E2 andthe reference electrode/insulator R/I can be varied to achieve a desiredsignal level, to enable the use of the desired voltage (e.g., to biasthe sensor), and the like, as described herein.

In one exemplary embodiment, a diffusion barrier D (described in greaterdetail below) separates the working electrodes. The diffusion barriercan be spatial, physical, or temporal. For example, the distance aroundthe reference electrode (e.g., from the first working electrode E1 tothe second working electrode E2, around a portion of the circumferenceof the reference electrode R) acts as a spatial diffusion barrier. Inone exemplary embodiment, the working electrodes are coated with a layerof insulating material I (e.g., non-conductive material or dielectric)to prevent direct contact between the working electrodes E1, E2 and thereference electrode R. A portion of the insulator I on an exteriorsurface of each working electrode is etched away, to expose theelectrode's electroactive surface. In some embodiments, an enzymesolution (e.g., containing active GOx) is applied to the electroactivesurfaces of both electrodes, and dried. Thereafter, the enzyme appliedto one of the electroactive surfaces is inactivated. As is known in theart, enzymes can be inactivated by a variety of means, such as heat,treatment with inactivating (e.g., denaturing) solvents, proteolysis,laser irradiation or UV irradiation (e.g., at 254-320 nm). For example,the enzyme coating one of the electroactive surfaces can be inactivatedby masking one of the electroactive surfaces/electrodes (e.g., E1,temporarily covered with a UV-blocking material); irradiating the sensorwith UV light (e.g., 254-320 nm; a wavelength that inactivates theenzyme, such as by cross-linking amino acid residues) and removing themask. Accordingly, the GOx on E2 is inactivated by the UV treatment, butthe E1 GOx is still active due to the protective mask. In otherembodiments, an enzyme solution containing active enzyme is applied to afirst electroactive surface (e.g., E1) and an enzyme solution containingeither inactivated enzyme or no enzyme is applied to the secondelectroactive surface (e.g., E2). Accordingly, the enzyme-coated firstelectroactive surface (e.g., E1) detects analyte-related signal andnon-analyte-related signal; while the second electroactive surface(e.g., E2), which lacks active enzyme, detects non-analyte-relatedsignal. As described herein, the sensor electronics can use the datacollected from the two working electrodes to calculate the analyte-onlysignal.

Although this exemplary embodiment illustrates one embodiment of anintegrally-formed sensor having a diffusion barrier D, one skilled inthe art appreciates a variety of alternative configurations, such as butnot limited to the embodiment shown in FIG. 7I. In this exemplaryembodiment, the reference electrode is formed of at least two adjacentpieces shaped such that the working electrodes fill at least some spacebetween them. The at least two pieces can be any shape known in the art,as described herein. In some embodiments, the at least two pieces aresymmetrical and/or mirror images of each other, but one skilled in theart will recognize that this is not a requirement. In variousembodiments, an insulating material can be coated on the workingelectrodes and/or the reference electrode(s) to prevent contact therebetween. As described elsewhere herein, the working electrodes candetect the same analyte or separate analytes, or one of the workingelectrodes may act as a counter electrode (e.g., auxiliary electrode).Although this exemplary embodiment illustrates one example of a sensorhaving a reference electrode R that is formed of at least two piecesshaped such that the working electrodes fill at least some space betweenthe pieces, one skilled in the art appreciates that a variety of sensorconfigurations are possible. For example, the reference electrode can beformed of three or more pieces. In other example, the sensor can beconfigured with more than two working electrodes (e.g., 3, 4, or 5working electrodes, or more).

FIG. 7J is a schematic view of an integrally formed sensor in yetanother embodiment. In this exemplary embodiment, the referenceelectrode R is formed in any desired extruded geometry, such as anapproximate X-shape. Two or more working electrodes E1, E2 are disposedon substantially opposing sides of the reference electrode, with adiffusion barrier D between them. In this embodiment, the diffusionbarrier is a physical diffusion barrier, namely the distance between thetwo working electrodes (e.g., around the reference electrode). In someembodiments, the electrodes are bundled and held together by a wrappingof wire or fiber. In other embodiments, the electrodes are twistedaround the lengthwise axis of the extruded X-shaped reference electrode,to form a coaxial sensor. Although this exemplary embodiment illustratesan integrally formed sensor, one skilled in the art appreciates avariety of alternative configurations. For example, furthering someembodiments, three or four working electrodes can be disposed around thereference electrode (e.g., in the indentations between the legs/arms ofthe X-shaped electrode). In other embodiments, the reference electrodecan be Y-shapes, star-shaped, flower-shaped, scalloped, or any otherconvenient shape with multiple substantially isolated sides. In someembodiments, an insulating material I takes the place of the referenceelectrode of FIG. 7J, which is remotely located. In an alternativeembodiment, a working electrode is replaced with a counter electrode. Asdescribed elsewhere herein, the sensor components are bifunctional.Namely, the electrodes and reference electrode provide electricalconduction and the sensor's structure. The reference electrode (orinsulating material) provides a physical diffusion barrier D. Inaddition to providing shape to the sensor, the insulating material actsas insulator by preventing direct electrical contact between theelectrodes. Similarly, the materials selected to construct the sensordetermine the sensor's flexibility. As described elsewhere, activeenzyme is applied to the electroactive surface of at least one workingelectrode (e.g., E1). In some embodiments, no enzyme (or inactivatedenzyme) is applied to the electroactive surface of a second workingelectrode (e.g., E2). In an alternative embodiment, a second enzyme isapplied to the second working electrode (e.g., E2) such that the sensorcan measure the signals of two different analytes (e.g., glucose andaureate or oxygen). FIG. 7K is a schematic of a sensor in anotherembodiment. In some preferred embodiments, the sensor is configured tobe integrally formed of two working electrodes. In this exemplaryembodiment, the sensor includes two electrodes E1, E2 (e.g., metalwires), wherein each electrode is coated with a non-conductive materialI (e.g., and insulator). As is shown in FIG. 7K, the first workingelectrode E1 formed within the insulator I leaving space for an enzyme.For example, an enzyme solution 702 (e.g., GOx for detecting glucose) isdisposed within the space 701. In contrast, the second working electrodeE2 extends substantially flush with the insulator I. A membrane system703 coats the electrodes. A diffusion barrier D separates the workingelectrodes. In some embodiments, the first and second electrodes areseparated by a distance D that substantially prevents diffusion of H₂O₂from the first electrode (e.g., with active enzyme) to the secondelectrode (e.g., without active enzyme). Although this exemplaryembodiment illustrates one integrally formed sensor, one skilled in theart appreciates a variety of alternative configurations. For example,the use of more than two working electrodes and wrapping the constructwith a reference electrode wire R or disposing the reference electroderemotely from the sensor.

FIG. 7L is a schematic of a sensor in one embodiment. In some preferredembodiments, the sensor is designed to be integrally formed. In thisexemplary embodiment, two electrodes E1, E2 are embedded within aninsulator I. The sensor can be formed by embedding conductive wireswithin a dielectric, curing the dielectric and then cutting sensors ofthe desired length. The cut end provides the exposed electroactiveelectrode surfaces and can be polished or otherwise treated. Althoughthis exemplary embodiment illustrates one integrally formed sensor, oneskilled in the art appreciates a variety of alternative configurations.For example, additional electrode wires can be embedded in thedielectric material. In another example, a reference electrode (e.g.,wire or cylinder) can be coiled or wrapped around the sensor (e.g., onthe surface of the insulator). Alternatively, as described elsewhereherein, the reference electrode can be disposed remotely from theworking electrodes E1, E2, such as on the host's skin or on anotherportion of the sensor. One advantage of this configuration is that it isrelatively simple to embed electrode wires in a long cylinder ofinsulating material and then cut the sensors to any desired size and/orshape.

FIG. 7M is a schematic cross-sectional view of a sensor having multipleworking and reference electrodes, in one embodiment. In some preferredembodiments, the sensor is integrally formed. In this exemplaryembodiment, the sensor includes a plurality of working electrodes (e.g.,E1, E2, E3) that are layered with a plurality of reference electrodes(e.g., R1, R2, Rn). In some embodiments, the working electrodes arecoated with an insulating material to prevent direct contact withadjacent reference electrodes. In some embodiments, the referenceelectrodes are also coated with insulative material. In someembodiments, layers of insulating material separate the layers. In someembodiments, at least one of the working electrodes is a counterelectrode. As described herein, in some embodiments, electroactivesurfaces are exposed on one or more electrodes, such as by strippingaway a portion of an insulating coating, such as on the sides of thesensor. In other embodiments, an extended electrode structure (e.g., along sandwich of electrode layers) that is cut to the desired length,and the cut end includes the exposed electroactive surfaces of theelectrodes. An enzyme layer can be applied to one or more of theelectroactive surfaces, as described herein. Depending upon the desiredsensor function, the working electrodes can be configured to detect thesame analyte (e.g., all electroactive surfaces coated with GOx glucose)or different analytes (e.g., one working electrode detects glucose,another detects oxygen and the third detects ureate), as describedherein. Although this exemplary embodiment illustrates a sensor having aplurality of working and reference electrodes, one skilled in the artappreciates a variety of alternative configurations. For example, insome embodiments, the electrodes can be of various sizes, depending upontheir purpose. For example, in one sensor, it may be preferred to use a3 mm oxygen electrode, a 10 mm glucose electrode and a 4 mm counterelectrode, all separated by reference electrodes. In another embodiment,each reference electrode can be functionally paired with a workingelectrode. For example, the electrodes can be pulsed on and off, suchthat a first reference electrode R1 is active only when the firstworking electrode E1 is active, and a second reference electrode R2 isactive only when the second working electrode E2 is active. In anotherembodiment, a flat sensor (e.g., disk-shaped) can be manufactured bysandwiching reference electrodes between working electrodes, cutting thesandwich into a cylinder, and the cutting the cylinder cross-wise(perpendicularly or at an angle) into disks.

FIG. 7N is a schematic cross-sectional view of the manufacture of anintegrally formed sensor, in one embodiment. In some preferredembodiments, at least two working electrodes (E1, E2) and optionally areference electrode R are embedded in a quantity 704 of insulatingmaterial I. The working electrodes are separated by a diffusion barrierD. After the insulator has been cured (e.g., vulcanized or solidified)the structure is shaped (e.g., carved, scraped or cut etc.) to the finalsensor shape 705, such that excess insulation material is removed. Insome embodiments, multiple sensors can be formed as an extendedstructure of electrode wires embedded in insulator, which issubsequently cut to the desired length, wherein the exposed electrodeends (e.g., at the cut surface) become the electroactive surfaces of theelectrodes. In other embodiments, portions of the insulator adjacent tothe electrodes (e.g., windows) can be removed (e.g., by cutting orscraping, etc.) to expose the electroactive surfaces. Depending upon thesensor's configuration and purpose, an enzyme solution can be applied toone or more of the electroactive surfaces, as described elsewhereherein. Although this exemplary embodiment illustrates one technique ofmanufacturing a sensor having insulation-embedded electrodes, oneskilled in the art appreciates a variety of alternative configurations.For example, a diffusion barrier D, can comprise both the referenceelectrode R and the insulating material I, or only the referenceelectrode. In another example, windows exposing the electroactivesurfaces can be formed adjacent to each other (e.g., on the same side ofthe reference electrode) or on opposite sides of the referenceelectrode. Still, in other embodiments, more working or referenceelectrodes can be included, and the working and reference electrodes canbe of relatively larger or smaller size, depending upon the sensor'sconfiguration and operating requirements (e.g., voltage and/or currentrequirements).

FIGS. 8A and 8B are schematic views of a sensor in yet anotherembodiment. FIG. 8A is a view of the cross-section and side of an invivo portion of the sensor. FIG. 8B is a side view of the ex vivoportion of the sensor (e.g., the portion that is connected to the sensorelectronics, as described elsewhere herein). Namely, two workingelectrodes E1, E2 that are coated with insulator I and then disposed onsubstantially opposing sides of a reference electrode R, such as asilver or silver/silver chloride electrode (see FIG. 8A). The workingelectrodes are separated by a diffusion barrier D that can include aphysical barrier (provided by the reference electrode and/or theinsulating material coatings), a spatial barrier (provided by staggeringthe electroactive surfaces of the working electrodes), or a temporalbarrier (provided by oscillating the potentials between the electrodes).In some embodiments, the reference electrode R has a surface area atleast 6-times the surface area of the working electrodes. Additionally,the reference electrode substantially can act as a spatial diffusionbarrier between the working electrodes due to its larger size (e.g., thedistance across the reference electrode, from one working electrode toanother).

The electrodes can be held in position by wrapping with wire or anon-conductive fiber, a non-conductive sheath, a biointerface membranecoating, or the like. The electroactive surfaces of the workingelectrodes are exposed. In some embodiments, the end of the sensor iscut off, to expose the ends of the wires. In other embodiments, the endsof the wires are coated with insulating material; and the electroactivesurfaces are exposed by removing a portion of the insulating material(e.g., a window 802 cut into the side of the insulation coating theelectrode). In some embodiments, the windows exposing the electroactivesurfaces of the electrodes can be staggered (e.g., spaced such that oneor more electrodes extends beyond the other one or more electrodes),symmetrically arranged or rotated to any degree; for example, tosubstantially prevent diffusion of electroactive species from oneworking electrode (e.g., 802 a) to the other working electrode (e.g.,802 b), as will be discussed in greater detail elsewhere herein. Invarious embodiments, the reference electrode is not coated with anonconductive material. The reference electrode can have a surface areathat is at least 6 times the surface area of the exposed workingelectrode electroactive surfaces. In some embodiments, the referenceelectrode R surface area is 7-10 times (or larger) than the surface areaof the working electrode electroactive surfaces. In still otherembodiments, the reference electrode can be only 1-5 times the surfacearea of working electrode electroactive surfaces (e.g., (E1+E2)×1=R or(E1+E2)×2=R, etc.).

The ex vivo end of the sensor is connected to the sensor electronics(not shown) by electrical connectors 804 a, 804 b, 804 c. In someembodiments, the ex vivo end of the sensor is stepped. For example, theex vivo end of the reference electrode R terminates within electricalconnector 804 a. The ex vivo end of the first working electrode E1 isexposed (e.g., nonconductive material removed therefrom) and terminatesa small distance past the reference electrode R, within electricalconnector 804 b. Similarly, the ex vivo end of the second workingelectrode E2 is exposed (e.g., nonconductive material removed therefrom)and terminates a small distance past the termination of the firstworking electrode E1, within electrical connector 804 c.

Although this exemplary embodiment illustrates one configuration of anintegrally formed sensor, one skilled in the art appreciates a varietyof alternative configurations. For example, in some embodiments, aportion of the in vivo portion of the sensor can be twisted and/orstepped. More working, reference, and/or counter electrodes, as well asinsulators, can be included. The electrodes can be of relatively largeror smaller size, depending upon the sensor's intended function. In someembodiments, the electroactive surfaces can be staggered. In still otherembodiments, the reference electrode can be disposed remotely from thesensor, as described elsewhere herein. For example, the referenceelectrode shown in FIG. 8A can be replaced with a non-conductive supportand the reference electrode disposed on the host's skin.

With reference to the ex vivo portion of the sensor, one skilled in theart appreciates additional alternative configurations. For example, inone embodiment, a portion of the ex vivo portion of the sensor can betwisted or coiled. In some embodiments, the working and referenceelectrodes can be of various lengths and configurations not shown inFIG. 8B. For example, the reference electrode R can be the longest(e.g., connect to electrical contact 804 c) and the first second workingelectrode E2 can be the shortest (e.g., connect to electrical contact804 a). In other embodiments, the first working electrode E1 may beeither the longest electrode (e.g., connect to electrical contact 804 c)or the shortest electrode (e.g., connect to electrical contact 804 a).

FIG. 9A is a schematic view that illustrates yet another exemplaryembodiment of an integrally formed analyte sensor. Namely, two workingelectrodes E1, E2 are bundled together and substantially encircled witha cylindrical silver or silver/silver chloride reference electrode R (orthe like). The reference electrode can be crimped at a location 902, toprevent movement of the working electrodes E1, E2 within the referenceelectrode R cylinder. In alternative embodiments, a reference electrodecan be rolled or coiled around the working electrodes E1, E2, to formthe reference electrode R. Preferably, the working electrodes are atleast partially insulated as described in more detail elsewhere herein;such as by coating with a non-conductive material, such as but notlimited to Parylene. One skilled in the art appreciates that a varietyof alternative configurations are possible.

FIG. 9B illustrates another embodiment of an integrally formed analytesensor. Namely, two working electrodes E1, E2 are bundled together witha silver or silver/silver chloride wire reference electrode R coiledthere around. The reference electrode can be coiled tightly, to preventmovement of the working electrodes E1, E2 within the reference electrodeR coil.

Referring again to FIGS. 9A to 9B, near the tip of the in vivo portionof the sensor, windows 904 a and 904 b are formed on the workingelectrodes E1, E2. Portions of the non-conductive material (e.g.,insulator) coating each electrode is removed to form windows 904 a and904 b. The electroactive surfaces of the electrodes are exposed viawindows 904 a and 904 b. As described elsewhere herein, the electrodeelectroactive surfaces exposed through windows 904 a and 904 b arecoated with a membrane system. An active enzyme (e.g., GOx is used ifglucose is the analyte) is disposed within or beneath or within themembrane covering one of the windows (e.g., 904 a or 904 b). Themembrane covering the other window can include inactivated enzyme (e.g.,GOx inactivated by heat, solvent, UV or laser irradiation, etc., asdescribed herein) or no enzyme. The electrode having active enzymedetects a signal related to the analyte concentration and non-analyterelated signal (e.g., due to background, etc.). In contrast, theelectrode having inactive enzyme or no enzyme detects substantially onlythe non-analyte related signal. These signals are transmitted to sensorelectronics (discussed elsewhere herein) to calculate an analyteconcentration based on only the signal component related to only theanalyte (described elsewhere herein).

In general, the windows 904 a and 904 b are separated or staggered by adistance D, which is selected to be sufficiently large thatelectroactive species (e.g., H₂O₂) do not substantially diffuse from onewindow to the other (e.g., from 904 a to 904 b). In an exemplaryembodiment of a glucose-oxidase-based sensor, active enzyme is includedin the membrane covering window 904 a and inactive enzyme is included inthe membrane covering window 904 b. Distance D is configured to be largeenough that H₂O₂ cannot diffuse from window 904 a to window 904 b, whichlacks active enzyme (as discussed elsewhere herein). In someembodiments, the distance D is at least about 0.020 inches or less toabout 0.120 inches or more. In some embodiments, D is at least about0.030 to about 0.050 inches. In other embodiments, D is at least about0.090 to about 0.095 inches. One skilled in the art appreciatesalternative embodiments of the diffusion barrier D. Namely, thediffusion barrier D can be spatial (discussed herein with relation toFIGS. 9A and 9B), physical or temporal (see discussion of DiffusionBarriers herein and FIG. 10). In some embodiments, a physical diffusionbarrier D, such as but not limited to an extended non-conductivestructure placed between the working electrodes (e.g., FIG. 8A),substantially prevents diffusion of H₂O₂ from one working electrode(having active enzyme) to another working electrode (having no activeenzyme). In other embodiments, a temporal diffusion barrier D is createdby pulsing or oscillating the electrical potential, such that only oneworking electrode is activated at a time.

In various embodiments, one of the windows 904 a or 904 b comprises anenzyme system configured to detect the analyte of interest (e.g.,glucose or oxygen). The other window comprises no active enzyme system(e.g., wherein the enzyme system lacks enzyme or wherein the enzyme hasbeen de-activated). In some embodiments, wherein the “enzyme systemlacks enzyme,” a layer may be applied, similar to an active enzymelayer, but without the actual enzyme included therein. In someembodiments, wherein “the enzyme has been de-activated” the enzyme canbe inactivated (e.g., by heat or solvent) prior to addition to theenzyme system solution or the enzyme can be inactivated afterapplication to the window.

In one exemplary embodiment, an enzyme is applied to both windows 904 aand 904 b followed by deactivation of the enzyme in one window. Forexample, one window can be masked (e.g., to protect the enzyme under themask) and the sensor then irradiated (to deactivate the enzyme in theunmasked window). Alternatively, one of the enzyme-coated windows (e.g.,the first window but not the second window) can be sprayed or dipped inan enzyme-deactivating solvent (e.g., treated with a protic acidsolution such a hydrochloric acid or sulfuric acid). For example, awindow coated with GOx can be dipped in dimethyl acetamide (DMAC),ethanol, or tetrahydrofuran (THF) to deactivate the GOx. In anotherexample, the enzyme-coated window can be dipped into a hot liquid (e.g.,water or saline) to deactivate the enzyme with heat.

In these embodiments, the design of the active and inactive enzymewindow is at least partially dependent upon the sensor's intended use.In some embodiments, it is preferred to deactivate the enzyme coated onwindow 904 a. In other embodiments, it is preferred to deactivate theenzyme coated on window 904 b. For example, in the case of a sensor tobe used in a host's blood stream, the choice depends upon whether thesensor will be inserted pointing upstream (e.g., against the blood flow)or pointing downstream (e.g., with the blood flow).

In one exemplary embodiment, an intravascular sensor is inserted intothe host's vein pointing upstream (against the blood flow), an enzymecoating on electrode E1 (window 904 a) is inactivated (e.g., by dippingin THF and rinsing) and an enzyme coating on electrode E2 (in window 904b) is not inactivated (e.g., by not dipping in THF). Because the enzymeon the first electrode E1 (e.g., in window 904 a) is inactive,electroactive species (e.g., H₂O₂) will not be substantially generatedat window 904 a (e.g., the first electrode E1 generates substantially noH₂O₂ to effect the second electrode E2). In contrast, the active enzymeon the second electrode E2 (in window 904 b) generates H₂O₂ which atleast partially diffuses down stream (away from the windows) and thushas no effect on the first electrode E1, other features and advantagesof spatial diffusion barriers are described in more detail elsewhereherein.

In another exemplary embodiment, an intravascular sensor is insertedinto the host's vein pointing downstream (with the blood flow), theenzyme coating on electrode E1 (window 904 a) is active and the enzymecoating on electrode E2 (in window 904 b) is inactive. Because window904 a is located farther downstream than window 904 b, the H₂O₂ producedby the enzyme in 904 a diffuses downstream (away from window 904 b), andtherefore does not affect substantially electrode E2. In a preferredembodiment, the enzyme is GOx, and the sensor is configured to detectglucose. Accordingly, H₂O₂ produced by the GOx in window 904 a does notaffect electrode E2, because the sensor is pointing downstream and theblood flow carries away the H₂O₂ produced on electrode E1.

FIGS. 9A and 9B illustrate two embodiments of a sensor having a steppedsecond end (e.g., the back end, distal end or ex vivo end, describedwith reference to FIG. 8B) that connects the sensor to the sensorelectronics. Namely, each electrode terminates within an electricalconnector 804 such as but not limited to an elastomeric electricalconnector. Additionally, each electrode is of a different length, suchthat each electrode terminates within one of a plurality of sequentialelectrical connectors. For example, with reference to FIG. 9A, thereference electrode R is the shortest in length and terminates withinthe first electrical connector 804. The first working electrode E1 islonger than the reference electrode R, and terminates within the secondelectrical connector 804. Finally, the second working electrode E2 isthe longest electrode and terminates within the third electricalconnector 804. One skilled in the art appreciates that otherconfigurations are possible. For example, the first working electrode E1can be longer than the second working electrode E2. Accordingly, thesecond working electrode E2 would terminate within the second (e.g.,middle) electrical connector 804 and the first working electrode E1would terminate within the third (e.g., last) electrical connector 804.With reference to FIG. 9B, additional stepped second end configurationsare possible. In alternative embodiments, the second ends of the sensormay be separated from each other to connect to non-parallel,non-sequential electrical connectors.

FIG. 11 is a schematic view of a sensor in yet another embodiment. Inpreferred embodiments, the sensor is integrally formed, coaxial, and hasa stepped ex vivo end (e.g., back or second end). Electrodes E1, E2 andE3 are twisted to form a helix, such as a triple helix. Additionally, atthe back end of the sensor, the electrodes are stepped and eachelectrode is individually connected to the sensor electronics by anelectrical connector 804. At each electrode's second end, the electrodeengages an electrical connector 804 that joins the electrode to thesensor electronics. For example, the second end of electrode E1electrically connects electrical connector 1106. Similarly, the secondend of electrode E2 electrically connects electrical connector 1108 andthe second end of electrode E3 electrically connects electricalconnector 1110. As described elsewhere herein, each sensor component isdifunctional, and provides electrical conductance, structural support, adiffusion barrier, or insulation (see description elsewhere herein).Although this exemplary embodiment illustrates an integrally formed,coaxial sensor having a stepped back end, one skilled in the artappreciates a variety of alternative configurations. For example, one ofthe electrodes E1, E2 or E3 can be a reference electrode, or thereference electrode can be disposed remotely from the sensor, such asbut not limited to on the host's skin. In another example, the sensorcan have only two electrodes or more than three electrodes.

One skilled in the art recognizes a variety of alternativeconfigurations for the embodiments described herein. For example, in anyembodiment of an analyte sensor, the reference electrode (and optionallya counter electrode) can be disposed remotely from the workingelectrodes. For example, in FIGS. 7A1 through 9B and FIG. 11, thereference electrode R can be replaced with a non-conductive material,such as an insulator I. Depending upon the sensor's configuration andlocation of use, the reference electrode R can then be inserted into thehost in a location near to the sensor, applied to the host's skin, bedisposed within a fluid connector, be disposed on the ex-vivo portion ofthe sensor or even disposed on the exterior of the sensor electronics.

FIG. 7L illustrates an embodiment in which the reference and/or counterelectrode is located remotely from the first and second workingelectrodes E1 and E2, respectively. In one exemplary embodiment, thesensor is a needle-type sensor such as described with reference to FIG.1B, and the working electrodes E1, E2 are integrally formed togetherwith a substantially X-shaped insulator I and the reference electrode(and/or counter electrode) is placed on the host's skin (e.g., a button,plate, foil or wire, such as under the housing) or implantedtranscutaneously in a location separate from the working electrodes.

As another example, in one embodiment of a sensor configured to measurea host's blood, such as described in co-pending U.S. patent applicationSer. No. 11/543,396, filed on Oct. 4, 2006 and entitled “ANALYTESENSOR”, and which is incorporated herein by reference in its entirety;one or more working electrodes can be inserted into the host's blood viaa catheter and the reference and/or counter electrode can be placedwithin the a fluid connector (on the sensor) configured to be in fluidcommunication with the catheter; in such an example, the referenceand/or counter electrode is in contact with fluid flowing through thefluid connector but not in direct contact with the host's blood. Instill other embodiments, the reference and/or counter electrodes can beplaced exterior to the sensor, in bodily contact for example.

With reference to the analyte sensor embodiments disclosed herein, thesurface area of the electroactive portion of the reference (and/orcounter) electrode is at least six times the surface area of one or moreworking electrodes. In other embodiments, the reference (and/or counter)electrode surface is 1, 2, 3, 4, 5, 7, 8, 9 or 10 times the surface areaof the working electrodes. In other embodiments, the reference (and/orcounter) electrode surface area is 11, 12, 13, 14, 15, 16, 17, 18, 19 or20 times the surface area of the working electrodes. For example, in aneedle-type glucose sensor, similar to the embodiment shown in FIG. 1B,the surface area of the reference electrode (e.g., 18 or 20) includesthe exposed surface of the reference electrode, such as but not limitedto the electrode surface facing away from the working electrode 16.

In various embodiments, the electrodes can be stacked or grouped similarto that of a leaf spring configuration, wherein layers of electrode andinsulator (or individual insulated electrodes) are stacked in offsetlayers. The offset layers can be held together with bindings ofnon-conductive material, foil, or wire. As is appreciated by one skilledin the art, the strength, flexibility, and/or other material property ofthe leaf spring-configured or stacked sensor can be either modified(e.g., increased or decreased), by varying the amount of offset, theamount of binding, thickness of the layers, and/or materials selectedand their thicknesses, for example.

In some embodiments, the sensor (e.g., a glucose sensor) is configuredfor implantation into the host. For example, the sensor may be whollyimplanted into the host, such as but not limited to in the host'ssubcutaneous tissue (e.g., the embodiment shown in FIG. 1A). In otherembodiments, the sensor is configured for transcutaneous implantation inthe host's tissue. For example, the sensor can have a portion that isinserted through the host's skin and into the underlying tissue, andanother portion that remains outside the host's body (e.g., such asdescribed in more detail with reference to FIG. 1B). In still otherembodiments, the sensor is configured for indwelling in the host's bloodstream. For example, a needle-type sensor can be configured forinsertion into a catheter dwelling in a host's vein or artery. Inanother example, the sensor can be integrally formed on the exteriorsurface of the catheter, which is configured to dwell within a host'svein or artery. Examples of indwelling sensors can be found inco-pending U.S. patent application Ser. No. 11/543,396 filed on Oct. 4,2006 and entitled “ANALYTE SENSOR.” In various embodiments, the in vivoportion of the sensor can take alternative configurations, such as butnot limited to those described in more detail with reference to FIGS.7A-9B and 11.

In preferred embodiments, the analyte sensor substantially continuouslymeasures the host's analyte concentration. In some embodiments, forexample, the sensor can measure the analyte concentration every fractionof a second, about every fraction of a minute or every minute. In otherexemplary embodiments, the sensor measures the analyte concentrationabout every 2, 3, 4, 5, 6, 7, 8, 9, or 10 minutes. In still otherembodiments, the sensor measures the analyte concentration everyfraction of an hour, such as but not limited to every 15, 30 or 45minutes. Yet in other embodiments, the sensor measures the analyteconcentration about every hour or longer. In some exemplary embodiments,the sensor measures the analyte concentration intermittently orperiodically. In one preferred embodiment, the analyte sensor is aglucose sensor and measures the host's glucose concentration about every4-6 minutes. In a further embodiment, the sensor measures the host'sglucose concentration every 5 minutes.

In one exemplary embodiment, the analyte sensor is a glucose sensorhaving a first working electrode configured to generate a first signalassociated with both glucose and non-glucose related electroactivecompounds that have a first oxidation potential. Non-glucose relatedelectroactive compounds can be any compound, in the sensor's localenvironment that has an oxidation potential substantially overlappingwith the oxidation potential of H₂O₂, for example. While not wishing tobe bound by theory, it is believed that the glucose-measuring electrodecan measure both the signal directly related to the reaction of glucosewith GOx (produces H₂O₂ that is oxidized at the working electrode) andsignals from unknown compounds that are in the extracellular milieusurrounding the sensor. These unknown compounds can be constant ornon-constant (e.g., intermittent or transient) in concentration and/oreffect. In some circumstances, it is believed that some of these unknowncompounds are related to the host's disease state. For example, it isknow that blood chemistry changes dramatically during/after a heartattack (e.g., pH changes, changes in the concentration of various bloodcomponents/protein, and the like). Other compounds that can contributeto the non-glucose related signal are believed to be related to thewound healing process that is initiated by implantation/insertion of thesensor into the host, which is described in more detail with referenceto U.S. Patent Publication No. US-2007-0027370-A1, which is incorporatedherein by reference in its entirety. For example, transcutaneouslyinserting a needle-type sensor initiates a cascade of events thatincludes the release of various reactive molecules by macrophages.

In some embodiments, the glucose sensor includes a second (e.g.,auxiliary) working electrode that is configured to generate a secondsignal associated with non-glucose related electroactive compounds thathave the same oxidation potential as the above-described first workingelectrode (e.g., para supra). In some embodiments, the non-glucoserelated electroactive species includes at least one of interferingspecies, non-reaction-related H₂O₂, and other electroactive species. Forexample, interfering species includes any compound that is not directlyrelated to the electrochemical signal generated by the glucose-GOxreaction, such as but not limited to electroactive species in the localenvironment produces by other bodily processes (e.g., cellularmetabolism, wound healing, a disease process, and the like).Non-reaction-related H₂O₂ includes H₂O₂ from sources other than theglucose-GOx reaction, such as but not limited to H₂O₂ released by nearbycells during the course of the cells' metabolism, H₂O₂ produced by otherenzymatic reactions (e.g., extracellular enzymes around the sensor orsuch as can be released during the death of nearby cells or such as canbe released by activated macrophages), and the like. Other electroactivespecies includes any compound that has an oxidation potential similar toor overlapping that of H₂O₂.

The non-analyte (e.g., non-glucose) signal produced by compounds otherthan the analyte (e.g., glucose) obscured the signal related to theanalyte, contributes to sensor inaccuracy, and is considered backgroundnoise. As described in greater detail in the section entitled “NoiseReduction,” background noise includes both constant and non-constantcomponents and must be removed to accurately calculate the analyteconcentration. While not wishing to be bound by theory, it is believedthat the sensor of the preferred embodiments are designed (e.g., withsymmetry, coaxial design and/or integral formation) such that the firstand second electrodes are influenced by substantially the sameexternal/environmental factors, which enables substantially equivalentmeasurement of both the constant and non-constant species/noise. Thisadvantageously allows the substantial elimination of noise (includingtransient biologically related noise that has been previously seen toaffect accuracy of sensor signal due to it's transient and unpredictablebehavior) on the sensor signal (using electronics described elsewhereherein) to substantially reduce or eliminate signal effects due tonoise, including non-constant noise (e.g., unpredictable biological,biochemical species or the like) known to effect the accuracy ofconventional continuous sensor signals. Preferably, the sensor includeselectronics operably connected to the first and second workingelectrodes. The electronics are configured to provide the first andsecond signals that are used to generate glucose concentration datasubstantially without signal contribution due to non-glucose-relatednoise. Preferably, the electronics include at least a potentiostat thatprovides a bias to the electrodes. In some embodiments, sensorelectronics are configured to measure the current (or voltage) toprovide the first and second signals. The first and second signals areused to determine the glucose concentration substantially without signalcontribution due to non-glucose-related noise such as by but not limitedto subtraction of the second signal from the first signal or alternativedata analysis techniques. In some embodiments, the sensor electronicsinclude a transmitter that transmits the first and second signals to areceiver, where additional data analysis and/or calibration of glucoseconcentration can be processed. U.S. Patent Publication No.US-2005-0027463-A1, U.S. Patent Publication No. US-2005-0203360-A1, andU.S. Patent Publication No. US-2006-0036142-A1 describe systems andmethods for processing sensor analyte data and are incorporated hereinby reference in their entirety.

In preferred embodiments, the sensor electronics (e.g., electroniccomponents) are operably connected to the first and second workingelectrodes. The electronics are configured to calculate at least oneanalyte sensor data point. For example, the electronics can include apotentiostat, A/D converter, RAM, ROM, transmitter, and the like. Insome embodiments, the potentiostat converts the raw data (e.g., rawcounts) collected from the sensor to a value familiar to the host and/ormedical personnel. For example, the raw counts from a glucose sensor canbe converted to milligrams of glucose per deciliter of glucose (e.g.,mg/dl). In some embodiments, the electronics are operably connected tothe first and second working electrodes and are configured to processthe first and second signals to generate a glucose concentrationsubstantially without signal contribution due to non-glucose noiseartifacts. The sensor electronics determine the signals from glucose andnon-glucose related signal with an overlapping measuring potential(e.g., from a first working electrode) and then non-glucose relatedsignal with an overlapping measuring potential (e.g., from a secondelectrode). The sensor electronics then use these data to determine asubstantially glucose-only concentration, such as but not limited tosubtracting the second electrode's signal from the first electrode'ssignal, to give a signal (e.g., data) representative of substantiallyglucose-only concentration, for example. In general, the sensorelectronics may perform additional operations, such as but not limitedto data smoothing and noise analysis.

Bifunctionality

In some embodiments, the components of at least a portion (e.g., the invivo portion or the sensing portion) of the sensor possess bifunctionalproperties (e.g., provide at least two functions to the sensor). Theseproperties can include electrical conductance, insulative properties,structural support, and diffusion barrier properties.

In one exemplary embodiment, the analyte sensor is designed with twoworking electrodes, a membrane system and an insulating materialdisposed between the working electrodes. An active enzymatic membrane isdisposed above the first working electrode, while an inactive- ornon-enzymatic membrane is disposed above the second working electrode.Additionally, the working electrodes and the insulating material areconfigured provide at least two functions to the sensor, including butnot limited to electrical conductance, insulative properties, structuralsupport, and diffusion barrier. For example, in one embodiment of aglucose sensor, the two working electrodes support the sensor'sstructure and provide electrical conductance; the insulating materialprovides insulation between the two electrodes and provides additionalstructural support and/or a diffusional barrier.

In some embodiments, a component of the sensor is configured to provideboth electrical conductance and structural support. In an exemplaryembodiment, the working electrode(s) and reference electrode aregenerally manufactured of electrically conductive materials, such as butnot limited silver or silver/silver chloride, copper, gold, platinum,iridium, platinum-iridium, palladium, graphite, carbon, conductivepolymers, alloys, and the like. Accordingly, the electrodes are bothconductive and they give the sensor its shape (e.g., are supportive).

Referring to FIG. 1B, all three electrodes 16, 18, and 20 aremanufactured from plated insulator, a plated wire, or electricallyconductive material, such as but not limited to a metal wire.Accordingly, the three electrodes provide both electrical conductance(to measure glucose concentration) and structural support. Due to theconfiguration of the electrodes (e.g., the wires are about 0.001 inchesin diameter or less, to about 0.01 inches or more), the sensor isneedle-like and only about 0.003 inches or less to about 0.015 inches ormore.

Similarly, the electrodes of FIG. 7A through FIG. 9 provide electricalconductance, to detect the analyte of interest, as well as structuralsupport for the sensor. For example, the sensors depicted in FIGS. 7Athrough 7L embodiments that are substantially needle-like. Additionally,these sensors are substantially resilient, and therefore able to flex inresponse to mechanical pressure and then to regain their originalshapes. FIG. 7M depicts a cross-section of another sensor embodiment,which can be a composite (e.g., built up of layers of working andreference electrode materials) needle-like sensor or the composite“wire” can be cut to produce pancake-shaped sensors [describe itsbifunctionality without unnecessary characterizations (e.g., not“pancake-shaped”). FIG. 7N through FIG. 9 illustrate additional sensorembodiments, wherein the electrodes provide electrical conductance andsupport the sensor's needle-like shape.

In some embodiments, the first and second working electrodes areconfigured to provide both electrical conductance and structuralsupport. For example, in a needle-type sensor, the working electrodesare often manufactured of bulk metal wires (e.g., copper, gold,platinum, iridium, platinum-iridium, palladium, graphite, carbon,conductive polymers, alloys, and the like). The reference electrode,which can function as a reference electrode alone, or as a dualreference and counter electrode, are formed from silver or silver/silverchloride, or the like. The metal wires are conductive (e.g., can conductelectricity) and give the sensor its shape and/or structural support.For example, one electrode metal wire may be coiled around the otherelectrode metal wire (e.g., FIG. 1B or FIG. 7B). In a furtherembodiment, the sensor includes a reference electrode that is alsoconfigured to provide electrical conductance and structural support(e.g., FIG. 1B, FIGS. 7C to 7E). In general, reference electrodes aremade of metal, such as bulk silver or silver/silver chloride wires. Likethe two working electrodes, the reference electrode both conductselectricity and supports the structure of the sensor.

In some embodiments, the first and second working electrode and theinsulating material are configured provide at least two functions, suchas but not limited to electrical conductance, insulative properties,structural support, and diffusion barrier. As described elsewhereherein, the working electrodes are electrical conductors and alsoprovide support for the sensor. The insulating material (e.g., I) actsas an insulator, to prevent electrical communication between certainparts of the various electrodes. The insulating material also providesstructural support or substantially prevents diffusion of electroactivespecies from one working electrode to the other, which is discussed ingreater detail elsewhere herein.

In preferred embodiments, the sensor has a diffusion barrier disposedbetween the first and second working electrodes. The diffusion barrieris configured to substantially block diffusion of the analyte or aco-analyte (e.g., H₂O₂) between the first and second working electrodes.For example, a sheet of a polymer through which H₂O₂ cannot diffuse canbe interposed between the two working electrodes. Diffusion barriers arediscussed in greater detail elsewhere herein.

In some embodiments of the preferred embodiments, the analyte sensorincludes a reference electrode that is configured to provide electricalconductance and a diffusion barrier. Electrical conductance is aninherent property of the metal used to manufacture the referenceelectrode. However, the reference electrode can be configured to preventspecies (e.g., H₂O₂) from diffusing from the first working electrode tothe second working electrode. For example, a sufficiently largereference electrode can be placed between the two working electrodes. Insome embodiments, the reference electrode projects farther than the twoworking electrodes. In other embodiments, the reference electrode is sobroad that a substantial portion of the H₂O₂ produced at the firstworking electrode cannot diffuse to the second working electrode, andthereby significantly affect the second working electrode's function.

In a further embodiment, the reference electrode is configured toprovide a diffusion barrier and structural support. As describedelsewhere herein, the reference electrode can be constructed of asufficient size and/or shape that a substantial portion of the H₂O₂produced at a first working electrode cannot diffuse to the secondworking electrode and affect the second working electrode's function.Additionally, metal wires are generally resilient and hold their shape,the reference electrode can also provide structural support to thesensor (e.g., help the sensor to hold its shape).

In some embodiments of the analyte sensor described elsewhere herein,the insulating material is configured to provide both electricalinsulative properties and structural support. In one exemplaryembodiment, portions of the electrodes are coated with a non-conductivepolymer. Inherently, the non-conductive polymer electrically insulatesthe coated electrodes from each other, and thus substantially preventspassage of electricity from one coated wire to another coated wire.Additionally, the non-conductive material (e.g., a non-conductivepolymer or insulating material) can stiffen the electrodes and make themresistant to changes in shape (e.g., structural changes).

In some embodiments, a sensor component is configured to provideelectrical insulative properties and a diffusion barrier. In oneexemplary embodiment, the electrodes are coated with the non-conductivematerial that substantially prevents direct contact between theelectrodes, such that electricity cannot be conducted directly from oneelectrode to another. Due to the non-conductive coatings on theelectrodes, electrical current must travel from one electrode to anotherthrough the surrounding aqueous medium (e.g., extracellular fluid,blood, wound fluid, or the like). Any non-conductive material (e.g.,insulator) known in the art can be used to insulate the electrodes fromeach other. In exemplary embodiments, the electrodes can be coated withnon-conductive polymer materials (e.g., parylene, PTFE, ETFE,polyurethane, polyethylene, polyimide, silicone and the like) bydipping, painting, spraying, spin coating, or the like.

Non-conductive material (e.g., insulator, as discussed elsewhere herein)applied to or separating the electrodes can be configured to preventdiffusion of electroactive species (e.g., H₂O₂) from one workingelectrode to another working electrode. Diffusion of electroactivespecies from one working electrode to another can cause a false analytesignal. For example, electroactive species (e.g., H₂O₂) that are createdat a first working electrode having active enzyme (e.g., GOx) candiffuse to a nearby working electrode (e.g., without active GOx). Whenthe electroactive species arrives at the second working electrode, thesecond electrode registers a signal (e.g., as if the second workingelectrode comprised active GOx). The signal registered at the secondworking electrode due to the diffusion of the H₂O₂ is aberrant and cancause improper data processing in the sensor electronics. For example,if the second electrode is configured to measure a substantiallynon-analyte related signal (e.g., background) the sensor will record ahigher non-analyte related signal than is appropriate, possiblyresulting in the sensor reporting a lower analyte concentration thanactually is present in the host. This is discussed in greater detailelsewhere herein.

In preferred embodiments, the non-conductive material is configured toprovide a diffusion barrier and structural support to the sensor.Diffusion barriers are described elsewhere herein. Non-conductivematerials can be configured to support the sensor's structure. In some,non-conductive materials with relatively more or less rigidity can beselected. For example, if the electrodes themselves are relativelyflexible, it may be preferred to select a relatively rigidnon-conductive material, to make the sensor stiffer (e.g., less flexibleor bendable). In another example, if the electrodes are sufficientlyresilient or rigid, a very flexible non-conductive material may becoated on the electrodes to bind the electrodes together (e.g., keep theelectrodes together and thereby hold the sensor's shape).

Referring now to FIGS. 7C to 7J, the non-conductive material can becoated on or wrapped around the grouped or bundled electrodes, toprevent the electrodes from separating and also to prevent theelectrodes from directly touching each other. For example, withreference to FIG. 7C, each electrode can be individually coated by afirst non-conductive material and then bundled together. Then the bundleof individually insulated electrodes can be coated with a second layerof the first non-conductive material or with a layer or a secondnon-conductive material. In an embodiment of a sensor having thestructure shown in FIG. 7K, each electrode E1, E2 is coated with anon-conductive material/insulator I, and then coated with a secondnon-conductive material 703 (e.g., instead of a biointerface membrane).Similarly, in FIG. 7L, the non-conductive material I prevents electrodesE1 and E2 from making direct contact with each other as well as givingthe needle-like sensor its overall dimensions and shape.

FIG. 7N illustrates one method of configuring a sensor having anon-conductive material I that both provides electrical insulationbetween the electrodes E1, E2, R and provides structural support to thesensor. Namely, the electrodes are embedded in a non-conductive polymerI, which is subsequently vulcanized (704=before shaping). Aftervulcanization, the excess non-conductive polymer I is trimmed away(e.g., cutting or scraping, etc.) to produce a sensor having the finaldesired sensor shape 705=after shaping).

In some embodiments, a component of the sensor is configured to provideboth insulative properties and a diffusion barrier. Diffusion barriersare discussed elsewhere herein. In one exemplary embodiment, the workingelectrodes are separated by a non-conductive material/insulator that isconfigured such that electroactive species (e.g., H₂O₂) cannot diffusearound it (e.g., from a first electrode to a second electrode). Forexample, with reference to the embodiment shown in FIG. 7H, theelectrodes E1, E2 are placed in the groves carved into a cylinder ofnon-conductive material I. The distance D from E1 to E2 (e.g., around I)is sufficiently great that H₂O₂ produced at E1 cannot diffuse to E2 andthereby cause an aberrant signal at E2.

In some preferred embodiments, in addition to two working electrodes anda non-conductive material/insulator, the sensor includes at least areference or a counter electrode. In preferred embodiments, thereference and/or counter electrode, together with the first and secondworking electrodes, integrally form at least a portion of the sensor. Insome embodiments, the reference and/or counter electrode is locatedremote from the first and second working electrodes. For example, insome embodiments, such as in the case of a transcutaneous sensor, thereference and/or counter electrodes can be located on the ex vivoportion of the sensor or reside on the host's skin, such as a portion ofan adhesive patch. In other embodiments, such as in the case of anintravascular sensor, the reference and/or counter electrode can belocated on the host's skin, within or on the fluid connector (e.g.,coiled within the ex vivo portion of the device and in contact withfluid within the device, such as but not limited to saline) or on theexterior of the ex vivo portion of the device. In preferred embodiments,the surface area of the reference and/or counter electrode is as leastsix times the surface area of at least one of the first and secondworking electrodes. In a further embodiment, the surface area of thereference and/or counter electrode is at least ten times the surfacearea of at least one of the first and second electrodes.

In preferred embodiments, the sensor is configured for implantation intothe host. The sensor can be configured for subcutaneous implantation inthe host's tissue (e.g., transcutaneous or wholly implantable).Alternatively, the sensor can be configured for indwelling in the host'sblood stream (e.g., inserted through an intravascular catheter orintegrally formed on the exterior surface of an intravascular catheterthat is inserted into the host's blood stream).

In some embodiments, the sensor is a glucose sensor that has a firstworking electrode configured to generate a first signal associated withglucose (e.g., the analyte) and non-glucose related electroactivecompounds (e.g., physiological baseline, interferents, and non-constantnoise) having a first oxidation potential. For example, glucose has afirst oxidation potential. The interferents have an oxidation potentialthat is substantially the same as the glucose oxidation potential (e.g.,the first oxidation potential). In a further embodiment, the glucosesensor has a second working electrode that is configured to generate asecond signal associated with noise of the glucose sensor. The noise ofthe glucose sensor is signal contribution due to non-glucose relatedelectroactive compounds (e.g., interferents) that have an oxidationpotential that substantially overlaps with the first oxidation potential(e.g., the oxidation potential of glucose, the analyte). In variousembodiments, the non-glucose related electroactive species include aninterfering species, non-reaction-related hydrogen peroxide, and/orother electroactive species.

In preferred embodiments, the glucose sensor has electronics that areoperably connected to the first and second working electrodes and areconfigured to provide the first and second signals to generate glucoseconcentration data substantially without signal contribution due tonon-glucose-related noise. For example, the sensor electronics analyzethe signals from the first and second working electrodes and calculatethe portion of the first electrode signal that is due to glucoseconcentration only. The portion of the first electrode signal that isnot due to the glucose concentration can be considered to be background,such as but not limited to noise.

In preferred embodiments, the glucose sensor has a non-conductivematerial (e.g., insulative material) positioned between the first andsecond working electrodes. The non-conductive material substantiallyprevents cross talk between the first and second working electrodes. Forexample, the electrical signal cannot pass directly from a firstinsulated electrode to a second insulated electrode. Accordingly, thesecond insulated electrode cannot aberrantly record an electrical signaldue to electrical signal transfer from the first insulated electrode.

In preferred embodiments, the first and second working electrodes andthe non-conductive material integrally form at least a portion of thesensor (e.g., a glucose sensor). The first and second working electrodesintegrally form a substantial portion of the sensor configured forinsertion in the host (e.g., the in vivo portion of the sensor). In afurther embodiment, the sensor (e.g., a glucose sensor) includes areference electrode that, in addition to the first and second workingelectrodes, integrally forms a substantial portion of the sensorconfigured for insertion in the host (e.g., the in vivo portion of thesensor). In yet a further embodiment, the sensor (e.g., a glucosesensor) has an insulator (e.g., non-conductive material), wherein thefirst and second working electrodes and the insulator integrally form asubstantial portion of the sensor configured for insertion in the host(e.g., the in vivo portion of the sensor).

In preferred embodiments, the sensor (e.g., a glucose sensor) includes adiffusion barrier configured to substantially block diffusion of theanalyte (e.g., glucose) or a co-analyte (e.g., H₂O₂) between the firstand second working electrodes. For example, as described with referenceto FIG. 10, a diffusion barrier D (e.g., spatial, physical and/ortemporal) blocks diffusion of a species (e.g., glucose and/or H₂O₂) fromthe first working electrode E1 to the second working electrode E2. Insome embodiments, the diffusion barrier D is a physical diffusionbarrier, such as a structure between the working electrodes that blocksglucose and H₂O₂ from diffusing from the first working electrode E1 tothe second working electrode E2. In other embodiments, the diffusionbarrier D is a spatial diffusion barrier, such as a distance between theworking electrodes that blocks glucose and H₂O₂ from diffusing from thefirst working electrode E1 to the second working electrode E2. In stillother embodiments, the diffusion barrier D is a temporal diffusionbarrier, such as a period of time between the activity of the workingelectrodes such that if glucose or H₂O₂ diffuses from the first workingelectrode E1 to the second working electrode E2, the second workingelectrode E2 will not substantially be influenced by the H₂O₂ from thefirst working electrode E1.

With reference to FIG. 7H, if the diffusion barrier is spatial, adistance D separates the working electrodes, such that the analyte orco-analyte substantially cannot diffuse from a first electrode E1 to asecond electrode E2. In some embodiments, the diffusion barrier isphysical and configured from a material that substantially preventsdiffusion of the analyte or co-analyte there through. Again referring toFIG. 7H, the insulator I and/or reference electrode R is configured froma material that the analyte or co-analyte cannot substantially passthrough. For example, H₂O₂ cannot substantially pass through asilver/silver chloride reference electrode. In another example, aparylene insulator can prevent H₂O₂ diffusion between electrodes. Insome embodiments, wherein the diffusion barrier is temporal, the twoelectrodes are activated at separate, non-overlapping times (e.g.,pulsed). For example, the first electrode E1 can be activated for aperiod of one second, followed by activating the second electrode E2three seconds later (e.g., after E1 has been inactivated) for a periodof one second.

In additional embodiments, a component of the sensor is configured toprovide both a diffusional barrier and a structural support, asdiscussed elsewhere herein. Namely, the diffusion barrier can beconfigured of a material that is sufficiently rigid to support thesensor's shape. In some embodiments, the diffusion barrier is anelectrode, such as but not limited to the reference and counterelectrodes (e.g., FIG. 7G to 7J and FIG. 8A). In other embodiments, thediffusion barrier is an insulating coating (e.g., parylene) on anelectrode (e.g., FIG. 7K to 7L) or an insulating structure separatingthe electrodes (e.g., FIG. 8A and FIG. 10).

One preferred embodiment provides a glucose sensor configured forinsertion into a host for measuring a glucose concentration in the host.The sensor includes a first working electrode configured to generate afirst signal associated with glucose and non-glucose relatedelectroactive compounds having a first oxidation potential. The sensoralso includes a second working electrode configured to generate a secondsignal associated with noise of the glucose sensor comprising signalcontribution due to non-glucose related electroactive compounds thathave an oxidation potential that substantially overlaps with the firstoxidation potential (e.g., the oxidation potential of H₂O₂).Additionally, the glucose sensor includes a non-conductive materiallocated between the first and second working electrodes. Each of thefirst working electrode, the second working electrode, and thenon-conductive material are configured to provide at least two functionsselected from the group consisting of: electrical conductance,insulative properties, structural support, and diffusion barrier.

In some embodiments of the glucose sensor, each of the first workingelectrode and the second working electrode are configured to provideelectrical conductance and structural support. For example, the metalplated wire of electrodes conducts electricity and helps maintain thesensor's shape. In a further embodiment, the glucose sensor includes areference electrode that is configured to provide electrical conductanceand structural support. For example, the silver/silver chloridereference electrode is both electrically conductive and supports thesensor's shape. In some embodiments of the glucose sensor includes areference electrode that is configured to provide electrical conductanceand a diffusion barrier. For example, the silver/silver chloridereference electrode can be configured as a large structure or protrudingstructure, which separates the working electrodes by the distance D(e.g., FIG. 7G). Distance “D” is sufficiently large that glucose and/orH₂O₂ cannot substantially diffuse around the reference electrode.Accordingly, H₂O₂ produced at a first working electrode does notsubstantially contribute to signal at a second working electrode. Insome embodiments of the glucose sensor includes a reference electrodethat is configured to provide a diffusion barrier and structuralsupport. In some embodiments of the glucose sensor, the non-conductivematerial is configured to provide electrical insulative properties andstructural support. For example, non-conductive dielectric materials caninsulate an electrode and can be sufficiently rigid to stiffen thesensor. In still other embodiments, the non-conductive material isconfigured to provide electrical insulative properties and a diffusionbarrier. For example, a substantially rigid, non-conductive dielectriccan coat the electrodes and provide support, as shown in FIG. 7L. Inother embodiments, the non-conductive material is configured to providediffusion barrier and structural support. For example, a dielectricmaterial can protrude between the electrodes, to act as a diffusionbarrier and provide support to the sensor's shape, as shown in FIG. 10.

Noise Reduction

In another aspect, the sensor is configured to reduce noise, includingnon-constant non-analyte related noise with an overlapping measuringpotential with the analyte. A variety of noise can occur when a sensorhas been implanted in a host. Generally, implantable sensors measure asignal (e.g., counts) that generally comprises at least two components,the background signal (e.g., background noise) and the analyte signal.The background signal is composed substantially of signal contributiondue to factors other than glucose (e.g., interfering species,non-reaction-related hydrogen peroxide, or other electroactive specieswith an oxidation potential that overlaps with the analyte orco-analyte). The analyte signal (e.g., glucose) is composedsubstantially of signal contribution due to the analyte. Consequently,because the signal includes these two components, a calibration isperformed in order to determine the analyte (e.g., glucose)concentration by solving for the equations y=mx+b, where the value of brepresents the background of the signal.

There are a variety of ways noise can be recognized and/or analyzed. Inpreferred embodiments, the sensor data stream is monitored, signalartifacts are detected, and data processing is based at least in part onwhether or not a signal artifact has been detected, such as described inU.S. Patent Publication No. US-2005-0043598-A1 and U.S. PatentPublication No. US-2007-0027370-A1, herein incorporated by reference intheir entirety.

Accordingly, if a sensor is designed such that the signal contributiondue to baseline and noise can be removed, then more accurate analyteconcentration data can be provided to the host or a healthcareprofessional.

One embodiment provides an analyte sensor (e.g., glucose sensor)configured for insertion into a host for measuring an analyte (e.g.,glucose) in the host. The sensor includes a first working electrodedisposed beneath an active enzymatic portion of a membrane on thesensor; a second working electrode disposed beneath an inactive- ornon-enzymatic portion of the membrane on the sensor; and electronicsoperably connected to the first and second working electrode andconfigured to process the first and second signals to generate ananalyte (e.g., glucose) concentration substantially without signalcontribution due to non-glucose related noise artifacts.

Referring now to FIG. 9B, in another embodiment, the sensor has a firstworking electrode E1 and a second working electrode E2. The sensorincludes a membrane system (not shown) covering the electrodes, asdescribed elsewhere herein. A portion of the membrane system on thefirst electrode contains active enzyme, which is depicted schematicallyas oval 904 a (e.g., active GOx). A portion of the membrane system onthe second electrode is non-enzymatic or contains inactivated enzyme,which is depicted schematically as oval 904 b (e.g., heat- orchemically-inactivated GOx or optionally no GOx). A portion of thesensor includes electrical connectors 804. In some embodiments, theconnectors 804 are located on an ex vivo portion of the sensor. Eachelectrode (e.g., E1, E2, etc.) is connected to sensor electronics (notshown) by a connector 804. Since the first electrode E1 includes activeGOx, it produces a first signal that is related to the concentration ofthe analyte (in this case glucose) in the host as well as other speciesthat have an oxidation potential that overlaps with the oxidationpotential of the analyte or co-analyte (e.g., non-glucose related noiseartifacts, noise-causing compounds, background). Since the secondelectrode E2 includes inactive GOx, it produces a second signal that isnot substantially related to the analyte or co-analyte. Instead, thesecond signal is substantially related to noise-causing compounds andother background noise. The sensor electronics process the first andsecond signals to generate an analyte concentration that issubstantially free of the non-analyte related noise artifacts.Elimination or reduction of noise (e.g., non-constant background) isattributed at least in part to the configuration of the electrodes inthe preferred embodiments, e.g., the locality of first and secondworking electrode, the symmetrical or opposing design of the first andsecond working electrodes, and/or the overall sizing and configurationof the exposed electroactive portions. Accordingly, the host is providedwith improved analyte concentration data, upon which he can make medicaltreatment decisions (e.g., if he should eat, if he should takemedication or the amount of medication he should take). Advantageously,in the case of glucose sensors, since the sensor can provide improvedquality of data, the host can be maintained under tighter glucosecontrol (e.g., about 80 mg/dl to about 120 mg/dl) with a reduced risk ofhypoglycemia and hypoglycemia's immediate complications (e.g., coma ordeath). Additionally, the reduced risk of hypoglycemia makes it possibleto avoid the long-term complications of hyperglycemia (e.g., kidney andheart disease, neuropathy, poor healing, loss of eye sight) byconsistently maintaining tight glucose control (e.g., about 80 mg/dl toabout 120 mg/dl).

In one embodiment, the sensor is configured to substantially eliminate(e.g., subtract out) noise due to mechanical factors. Mechanical factorsinclude macro-motion of the sensor, micro-motion of the sensor, pressureon the sensor, local tissue stress, and the like. Since both workingelectrodes are constructed substantially symmetrically and identically,and due to the sensor's small size, the working electrodes aresubstantially equally affected by mechanical factors impinging upon thesensor. For example, if a build-up of noise-causing compounds occurs(e.g., due to the host pressing upon and manipulating (e.g., fiddlingwith) the sensor, for example) both working electrodes will measure theresulting noise to substantially the same extend, while only one workingelectrode (the first working electrode, for example) will also measuresignal due to the analyte concentration in the host's body. The sensorthen calculates the analyte signal (e.g., glucose-only signal) byremoving the noise that was measured by the second working electrodefrom the total signal that was measured by the first working electrode.

Non-analyte related noise can also be caused by biochemical and/orchemical factors (e.g., compounds with electroactive acidic, amine orsulfhydryl groups, urea, lactic acid, phosphates, citrates, peroxides,amino acids (e.g., L-arginine), amino acid precursors or break-downproducts, nitric oxide (NO), NO-donors, NO-precursors or otherelectroactive species or metabolites produced during cell metabolismand/or wound healing). As with noise due to mechanical factors, noisedue to biochemical/chemical factors will impinge upon the two workingelectrodes of the preferred embodiments (e.g., with and without activeGOx) about the same extent, because of the sensor's small size andsymmetrical configuration. Accordingly, the sensor electronics can usethese data to calculate the glucose-only signal, as described elsewhereherein.

In one exemplary embodiment, the analyte sensor is a glucose sensor thatmeasures a first signal associated with both glucose and non-glucoserelated electroactive compounds having a first oxidation potential. Forexample, the oxidation potential of the non-glucose relatedelectroactive compounds substantially overlaps with the oxidationpotential of H₂O₂, which is produced according to the reaction ofglucose with GOx and subsequently transfers electrons to the firstworking electrode (e.g., E1; FIG. 10). The glucose sensor also measuresa second signal, which is associated with background noise of theglucose sensor. The background noise is composed of signal contributiondue to noise-causing compounds (e.g., interferents),non-reaction-related hydrogen peroxide, or other electroactive specieswith an oxidation potential that substantially overlaps with theoxidation potential of H₂O₂ (the co-analyte). The first and secondworking electrodes integrally form at least a portion of the sensor,such as but not limited to the in vivo portion of the sensor, asdiscussed elsewhere herein. Additionally, each of the first workingelectrode, the second working electrode, and a non-conductivematerial/insulator are configured provide at least two functions (to thesensor), such as but not limited to electrical conductance, insulativeproperties, structural support, and diffusion barrier (describedelsewhere herein). Furthermore, the sensor has a diffusion barrier thatsubstantially blocks diffusion of glucose or H₂O₂ between the first andsecond working electrodes.

Diffusion Barrier

Another aspect of the sensor is a diffusion barrier, to prevent anundesired species, such as H₂O₂ or the analyte, from diffusing betweenactive (with active enzyme) and inactive (without active enzyme)electrodes. In various embodiments, the sensor includes a diffusionbarrier configured to be physical, spatial, and/or temporal.

FIG. 10 is a schematic illustrating one embodiment of a sensor (e.g., aportion of the in vivo portion of the sensor, such as but not limited tothe sensor electroactive surfaces) having one or more components thatact as a diffusion barrier (e.g., prevent diffusion of electroactivespecies from one electrode to another). The first working electrode E1is coated with an enzyme layer 1000 comprising active enzyme. Forexample, in a glucose sensor, the first working electrode E1 is coatedwith glucose oxidase enzyme (GOx). A second working electrode E2 isseparated from the first working electrode E1 by a diffusion barrier D,such as but not limited to a physical diffusion barrier (e.g., either areference electrode or a layer of non-conductive material/insulator).The diffusion barrier can also be spatial or temporal, as discussedelsewhere herein.

Glucose and oxygen diffuse into the enzyme layer 1000, where they reactwith GOx, to produce gluconate and H₂O₂. At least a portion of the H₂O₂diffuses to the first working electrode E1, where it iselectrochemically oxidized to oxygen and transfers two electrons (e.g.,2e⁻) to the first working electrode E1, which results in a glucosesignal that is recorded by the sensor electronics (not shown). Theremaining H₂O₂ can diffuse to other locations in the enzyme layer or outof the enzyme layer (illustrated by the wavy arrows). Without adiffusion barrier D, a portion of the H₂O₂ can diffuse to the secondworking electrode E2, which results in an aberrant signal that can berecorded by the sensor electronics as a non-glucose related signal(e.g., background).

Preferred embodiments provide for a substantial diffusion barrier Dbetween the first and second working electrodes (E1, E2) such that theH₂O₂ cannot substantially diffuse from the first working electrode E1 tothe second working electrode E2. Accordingly, the possibility of anaberrant signal produced by H₂O₂ from the first working electrode E1 (atthe second working electrode E2) is reduced or avoided.

In some alternative embodiments, the sensor is provided with a spatialdiffusion barrier between electrodes (e.g., the working electrodes). Forexample, a spatial diffusion barrier can be created by separating thefirst and second working electrodes by a distance that is too great forthe H₂O₂ to substantially diffuse between the working electrodes. Insome embodiments, the spatial diffusion barrier is about 0.010 inches toabout 0.120 inches. In other embodiments, the spatial diffusion barrieris about 0.020 inches to about 0.050 inches. Still in other embodiments,the spatial diffusion barrier is about 0.055 inches to about 0.095inches. A reference electrode R (e.g., a silver or silver/silverchloride electrode) or a non-conductive material I (e.g., a polymerstructure or coating such as Parylene) can be configured to act as aspatial diffusion barrier.

FIGS. 9A and 9B illustrate two exemplary embodiments of sensors withspatial diffusion barriers. In each embodiment, the sensor has twoworking electrodes E1 and E2. Each working electrode includes anelectroactive surface, represented schematically as windows 904 a and904 b, respectively. The sensor includes a membrane system (not shown).Over one electroactive surface (e.g., 904 a) the membrane includesactive enzyme (e.g., GOx). Over the second electroactive surface (e.g.,904 b) the membrane does not include active enzyme. In some embodiments,the portion of the membrane covering the second electroactive surfacecontains inactivated enzyme (e.g., heat- or chemically-inactivated GOx)while in other embodiments, this portion of the membrane does notcontain any enzyme (e.g., non-enzymatic). The electroactive surfaces 904a and 904 b are separated by a spatial diffusion barrier that issubstantially wide such that H₂O₂ produced at the first electroactivesurface 904 a cannot substantially affect the second electroactivesurface 904 b. In some alternative embodiments, the diffusion barriercan be physical (e.g., a structure separating the electroactivesurfaces) or temporal (e.g., oscillating activity between theelectroactive surfaces).

In another embodiment, the sensor is an indwelling sensor, such asconfigured for insertion into the host's circulatory system via a veinor an artery. In some exemplary embodiments, an indwelling sensorincludes at least two working electrodes that are inserted into thehost's blood stream through a catheter. The sensor includes at least areference electrode that can be disposed either with the workingelectrodes or remotely from the working electrodes. The sensor includesa spatial, a physical, or a temporal diffusion barrier. A spatialdiffusion barrier can be configured as described elsewhere herein, withreference to FIG. 7A through FIG. 8A.

FIG. 9B provides one exemplary embodiment of an indwelling analytesensor, such as but not limited to an intravascular glucose sensor to beused from a few hours to ten days or longer. Namely, the sensor includestwo working electrodes. One working electrode detects theglucose-related signal (due to active GOx applied to the electroactivesurface) as well as non-glucose related signal. The other workingelectrode detects only the non-glucose related signal (because no activeGOx is applied to its electroactive surface). H₂O₂ is produced on theworking electrode with active GOx. If the H₂O₂ diffuses to the otherworking electrode (the no GOx electrode) an aberrant signal will bedetected at this electrode, resulting in reduced sensor activity.Accordingly, it is desirable to separate the electroactive surfaces witha diffusion barrier, such as but not limited to a spatial diffusionbarrier. Indwelling sensors are described in more detail in copendingU.S. patent application Ser. No. 11/543,396 filed on Oct. 4, 2006 andentitled “ANALYTE SENSOR,” herein incorporated in its entirety byreference.

To configure a spatial diffusion barrier between the working electrodes,the location of the active enzyme (e.g., GOx) is dependent upon theorientation of the sensor after insertion into the host's artery orvein. For example, in an embodiment configured for insertion upstream inthe host's blood flow (e.g., against the blood flow), active GOx wouldbe applied to electroactive surface 904 b and inactive GOX (or no GOx)would be applied to electroactive surface 904 a (e.g., upstream from 904b, relative to the direction of blood flow). Due to this configuration,H₂O₂ produced at electroactive surface 904 b would be carrier downstream (e.g., away from electroactive surface 904 a) and thus not affectelectrode E1.

Alternatively, the indwelling electrode can also be configured forinsertion of the sensor into the host's vein or artery in the directionof the blood flow (e.g., pointing downstream). In this configuration,referred to as a spatial diffusion barrier, or as a flow path diffusionbarrier, the active GOx can be advantageously applied to electroactivesurface 904 a on the first working electrode E1. The electroactivesurface 904 b on the second working electrode E2 has no active GOx.Accordingly, H₂O₂ produced at electroactive surface 904 a is carriedaway by the blood flow, and has no substantial effect on the secondworking electrode E2.

In another embodiment of an indwelling analyte sensor, the referenceelectrode, which is generally configured of silver/silver chloride, canextend beyond the working electrodes, to provide a physical barrieraround which the H₂O₂ generated at the electrode comprising active GOxcannot pass the other working electrode (that has active GOx). In someembodiments, the reference electrode has a surface area that is at leastsix times larger than the surface area of the working electrodes. Inother embodiments, a 2-working electrode analyte sensor includes acounter electrode in addition to the reference electrode. As isgenerally know in the art, the inclusion of the counter electrode allowsfor a reduction in the reference electrode's surface area, and therebyallows for further miniaturization of the sensor (e.g., reduction in thesensor's diameter and/or length, etc.).

FIG. 7H provides one exemplary embodiment of a spatial diffusionbarrier, wherein the reference electrode/non-conductive insulatingmaterial R/I is sized and shaped such that H₂O₂ produced at the firstworking electrode E1 (e.g., with enzyme) does not substantially diffusearound the reference electrode/non-conductive material R/I to the secondworking electrode E2 (e.g., without enzyme). In another example, shownin FIG. 7J, the X-shaped the reference electrode/non-conductive materialR/I substantially prevents diffusion of electroactive species from thefirst working electrode E1 (e.g., with enzyme) to the second workingelectrode E2 (e.g., without enzyme). In another embodiment, such as thesensor shown in FIG. 7A, the layer of non-conductive material I (betweenthe electrodes) is of a sufficient length that the H₂O₂ produced at oneelectrode cannot substantially diffuse to another electrode. (e.g., fromE1 to either E2 or E3; or from E2 to either E1 or E3, etc.).

In some embodiments, a physical diffusion barrier is provided by aphysical structure, such as an electrode, insulator, and/or membrane.For example, in the embodiments shown in FIGS. 7G to 7J, the insulator(I) or reference electrode (R) act as a diffusion barrier. As anotherexample, the diffusion barrier can be a bioprotective membrane (e.g., amembrane that substantially resists or blocks the transport of a species(e.g., hydrogen peroxide), such as CHRONOTHANE®-H (apolyetherurethaneurea based on polytetramethylene glycol, polyethyleneglycol, methylene diisocyanate, and organic amines). As yet anotherexample, the diffusion barrier can be a resistance domain, as describedin more detail elsewhere herein; namely, a semipermeable membrane thatcontrols the flux of oxygen and an analyte (e.g., glucose) to theunderlying enzyme domain. Numerous other structures and membranes canfunction as a physical diffusion barrier as is appreciated by oneskilled in the art.

In other embodiments, a temporal diffusion barrier is provided (e.g.,between the working electrodes). By temporal diffusion barrier is meanta period of time that substantially prevents an electroactive species(e.g., H₂O₂) from diffusing from a first working electrode to a secondworking electrode. For example, in some embodiments, the differentialmeasurement can be obtained by switching the bias potential of eachelectrode between the measurement potential and a non-measurementpotential. The bias potentials can be held at each respective setting(e.g., high and low bias settings) for as short as milliseconds to aslong as minutes or hours. Pulsed amperometric detection (PED) is onemethod of quickly switching voltages, such as described in Bisenberger,M.; Brauchle, C.; Hampp, N. A triple-step potential waveform at enzymemultisensors with thick-film gold electrodes for detection of glucoseand sucrose. Sensors and Actuators 1995, B, 181-189, which isincorporated herein by reference in its entirety. In some embodiments,bias potential settings are held long enough to allow equilibration.

One preferred embodiment provides a glucose sensor configured forinsertion into a host for measuring glucose in the host. The sensorincludes first and second working electrodes and an insulator locatedbetween the first and second working electrodes. The first workingelectrode is disposed beneath an active enzymatic portion of a membraneon the sensor and the second working electrode is disposed beneath aninactive- or non-enzymatic portion of the membrane on the sensor. Thesensor also includes a diffusion barrier configured to substantiallyblock diffusion of glucose or hydrogen peroxide between the first andsecond working electrodes.

In a further embodiment, the glucose sensor includes a referenceelectrode configured integrally with the first and second workingelectrodes. In some embodiments, the reference electrode can be locatedremotely from the sensor, as described elsewhere herein. In someembodiments, the surface area of the reference electrode is at least sixtimes the surface area of the working electrodes. In some embodiments,the sensor includes a counter electrode that is integral to the sensoror is located remote from the sensor, as described elsewhere herein.

In a further embodiment, the glucose sensor detects a first signalassociated with glucose and non-glucose related electroactive compoundshaving a first oxidation potential (e.g., the oxidation potential ofH₂O₂). In some embodiments, the glucose sensor also detects a secondsignal is associated with background noise of the glucose sensorcomprising signal contribution due to interfering species,non-reaction-related hydrogen peroxide, or other electroactive specieswith an oxidation potential that substantially overlaps with theoxidation potential of hydrogen peroxide; the first and second workingelectrodes integrally form at least a portion of the sensor; and each ofthe first working electrode, the second working electrode and thenon-conductive material/insulator are configured provide at least twofunctions such as but not limited to electrical conductance, insulation,structural support, and a diffusion barrier

In further embodiments, the glucose sensor includes electronics operablyconnected to the first and second working electrodes. The electronicsare configured to calculate at least one analyte sensor data point usingthe first and second signals described above. In still another furtherembodiment, the electronics are operably connected to the first andsecond working electrode and are configured to process the first andsecond signals to generate a glucose concentration substantially withoutsignal contribution due to non-glucose noise artifacts.

Additional Membrane Configurations

Depending upon the sensor configuration, additional membraneconfigurations can be desirable. FIGS. 3A to 3B are cross-sectionalexploded schematic views of the sensing region of a glucose sensor 10,which show architectures of the membrane system 22 disposed overelectroactive surfaces of glucose sensors in some embodiments. In theillustrated embodiments of FIGS. 3A and 3B, the membrane system 22 ispositioned at least over the glucose-measuring working electrode 16 andthe optional auxiliary working electrode 18; however the membrane systemmay be positioned over the reference and/or counter electrodes 20, 20 ain some embodiments.

Reference is now made to FIG. 3A, which is a cross-sectional explodedschematic view of the sensing region in one embodiment wherein an activeenzyme 32 of the enzyme domain is positioned only over theglucose-measuring working electrode 16. In this embodiment, the membranesystem is formed such that the glucose oxidase 32 only exists above theglucose-measuring working electrode 16. In one embodiment, during thepreparation of the membrane system 22, the enzyme domain coatingsolution can be applied as a circular region similar to the diameter ofthe glucose-measuring working electrode 16. This fabrication can beaccomplished in a variety of ways such as screen-printing or padprinting. Preferably, the enzyme domain is pad printed during the enzymedomain fabrication with equipment as available from Pad Print Machineryof Vermont (Manchester, Vt.). This embodiment provides the active enzyme32 above the glucose-measuring working electrode 16 only, so that theglucose-measuring working electrode 16 (and not the auxiliary workingelectrode 18) measures glucose concentration. Additionally, thisembodiment provides an added advantage of eliminating the consumption ofO₂ above the counter electrode (if applicable) by the oxidation ofglucose with glucose oxidase.

FIG. 3B is a cross-sectional exploded schematic view of a sensing regionof the preferred embodiments, and wherein the portion of the activeenzyme within the membrane system 22 positioned over the auxiliaryworking electrode 18 has been deactivated 34. In one alternativeembodiment, the enzyme of the membrane system 22 may be deactivated 34everywhere except for the area covering the glucose-measuring workingelectrode 16 or may be selectively deactivated only over certain areas(for example, auxiliary working electrode 18, counter electrode 20 a,and/or reference electrode 20) by irradiation, heat, proteolysis,solvent, or the like. In such a case, a mask (for example, such as thoseused for photolithography) can be placed above the membrane that coversthe glucose-measuring working electrode 16. In this way, exposure of themasked membrane to ultraviolet light deactivates the glucose oxidase inall regions except that covered by the mask.

In some alternative embodiments, the membrane system is disposed on thesurface of the electrode(s) using known deposition techniques. Theelectrode-exposed surfaces can be inset within the sensor body, planarwith the sensor body, or extending from the sensor body. Although someexamples of membrane systems have been provided above, the conceptsdescribed herein can be applied to numerous known architectures notdescribed herein.

Sensor Electronics

In some embodiments, the sensing region may include reference and/orelectrodes associated with the glucose-measuring working electrode andseparate reference and/or counter electrodes associated with theoptional auxiliary working electrode(s). In yet another embodiment, thesensing region may include a glucose-measuring working electrode, anauxiliary working electrode, two counter electrodes (one for eachworking electrode), and one shared reference electrode. In yet anotherembodiment, the sensing region may include a glucose-measuring workingelectrode, an auxiliary working electrode, two reference electrodes, andone shared counter electrode. However, a variety of electrode materialsand configurations can be used with the implantable analyte sensor ofthe preferred embodiments.

In some alternative embodiments, the working electrodes areinterdigitated. In some alternative embodiments, the working electrodeseach comprise multiple exposed electrode surfaces; one advantage ofthese architectures is to distribute the measurements across a greatersurface area to overcome localized problems that may occur in vivo forexample, with the host's immune response at the biointerface.Preferably, the glucose-measuring and auxiliary working electrodes areprovided within the same local environment, such as described in moredetail elsewhere herein.

FIG. 4 is a block diagram that illustrates the continuous glucose sensorelectronics in one embodiment. In this embodiment, a first potentiostat36 is provided that is operatively associated with the glucose-measuringworking electrode 16. The first potentiostat 36 measures a current valueat the glucose-measuring working electrode and preferably includes aresistor (not shown) that translates the current into voltage. Anoptional second potentiostat 37 is provided that is operativelyassociated with the optional auxiliary working electrode 18. The secondpotentiostat 37 measures a current value at the auxiliary workingelectrode 18 and preferably includes a resistor (not shown) thattranslates the current into voltage. It is noted that in someembodiments, the optional auxiliary electrode can be configured to sharethe first potentiostat with the glucose-measuring working electrode. AnA/D converter 38 digitizes the analog signals from the potentiostats 36,37 into counts for processing. Accordingly, resulting raw data streams(in counts) can be provided that are directly related to the currentmeasured by each of the potentiostats 36 and 37.

A microprocessor 40, also referred to as the processor module, is thecentral control unit that houses EEPROM 42 and SRAM 44, and controls theprocessing of the sensor electronics. It is noted that certainalternative embodiments can utilize a computer system other than amicroprocessor to process data as described herein. In other alternativeembodiments, an application-specific integrated circuit (ASIC) can beused for some or all the sensor's central processing. The EEPROM 42provides semi-permanent storage of data, for example, storing data suchas sensor identifier (ID) and programming to process data streams (forexample, such as described in U.S. Patent Publication No.US-2005-0027463-A1, which is incorporated by reference herein in itsentirety. The SRAM 44 can be used for the system's cache memory, forexample for temporarily storing recent sensor data. In some alternativeembodiments, memory storage components comparable to EEPROM and SRAM maybe used instead of or in addition to the preferred hardware, such asdynamic RAM, non-static RAM, rewritable ROMs, flash memory, or the like.

A battery 46 is operably connected to the microprocessor 40 and providesthe necessary power for the sensor 10 a. In one embodiment, the batteryis a Lithium Manganese Dioxide battery, however any appropriately sizedand powered battery can be used (for example, AAA, Nickel-cadmium,Zinc-carbon, Alkaline, Lithium, Nickel-metal hydride, Lithium-ion,Zinc-air, Zinc-mercury oxide, Silver-zinc, and/or hermetically-sealed).In some embodiments the battery is rechargeable. In some embodiments, aplurality of batteries can be used to power the system. In someembodiments, one or more capacitors can be used to power the system. AQuartz Crystal 48 may be operably connected to the microprocessor 40 tomaintain system time for the computer system as a whole.

An RF Transceiver 50 may be operably connected to the microprocessor 40to transmit the sensor data from the sensor 10 to a receiver (see FIGS.4 and 5) within a wireless transmission 52 via antenna 54. Although anRF transceiver is shown here, some other embodiments can include a wiredrather than wireless connection to the receiver. In yet otherembodiments, the receiver can be transcutaneously powered via aninductive coupling, for example. A second quartz crystal 56 can providethe system time for synchronizing the data transmissions from the RFtransceiver. It is noted that the transceiver 50 can be substituted witha transmitter in other embodiments. In some alternative embodimentsother mechanisms such as optical, infrared radiation (IR), ultrasonic,or the like may be used to transmit and/or receive data.

Receiver

FIG. 5 is a schematic drawing of a receiver for the continuous glucosesensor in one embodiment. The receiver 58 comprises systems necessary toreceive, process, and display sensor data from the analyte sensor, suchas described in more detail elsewhere herein. Particularly, the receiver58 may be a pager-sized device, for example, and house a user interfacethat has a plurality of buttons and/or keypad and a liquid crystaldisplay (LCD) screen, and which may include a backlight. In someembodiments the user interface may also include a speaker, and avibrator such as described with reference to FIG. 6.

FIG. 6 is a block diagram of the receiver electronics in one embodiment.In some embodiments, the receiver comprises a configuration such asdescribed with reference to FIG. 5, above. However, the receiver maycomprise any reasonable configuration, including a desktop computer,laptop computer, a personal digital assistant (PDA), a server (local orremote to the receiver), or the like. In some embodiments, a receivermay be adapted to connect (via wired or wireless connection) to adesktop computer, laptop computer, a PDA, a server (local or remote tothe receiver), or the like in order to download data from the receiver.In some alternative embodiments, the receiver may be housed within ordirectly connected to the sensor in a manner that allows sensor andreceiver electronics to work directly together and/or share dataprocessing resources. Accordingly, the receiver, including itselectronics, may be generally described as a “computer system.”

A quartz crystal 60 may be operably connected to an RF transceiver 62that together function to receive and synchronize data streams via anantenna 64 (for example, transmission 52 from the RF transceiver 50shown in FIG. 4). Once received, a microprocessor 66 can process thesignals, such as described below.

The microprocessor 66, also referred to as the processor module, is thecentral control unit that provides the processing, such as storing data,calibrating sensor data, downloading data, controlling the userinterface by providing prompts, messages, warnings and alarms, or thelike. The EEPROM 68 may be operably connected to the microprocessor 66and provides semi-permanent storage of data, storing data such asreceiver ID and programming to process data streams (for example,programming for performing calibration and other algorithms describedelsewhere herein). SRAM 70 may be used for the system's cache memory andis helpful in data processing. For example, the SRAM stores informationfrom the continuous glucose sensor for later recall by the patient or adoctor; a patient or doctor can transcribe the stored information at alater time to determine compliance with the medical regimen or acomparison of glucose concentration to medication administration (forexample, this can be accomplished by downloading the information throughthe pc corn port 76). In addition, the SRAM 70 can also store updatedprogram instructions and/or patient specific information. In somealternative embodiments, memory storage components comparable to EEPROMand SRAM can be used instead of or in addition to the preferredhardware, such as dynamic RAM, non-static RAM, rewritable ROMs, flashmemory, or the like.

A battery 72 may be operably connected to the microprocessor 66 andprovides power for the receiver. In one embodiment, the battery is astandard AAA alkaline battery, however any appropriately sized andpowered battery can be used. In some embodiments, a plurality ofbatteries can be used to power the system. In some embodiments, a powerport (not shown) is provided permit recharging of rechargeablebatteries. A quartz crystal 84 may be operably connected to themicroprocessor 66 and maintains system time for the system as a whole.

A PC communication (com) port 76 can be provided to enable communicationwith systems, for example, a serial communications port, allows forcommunicating with another computer system (for example, PC, PDA,server, or the like). In one exemplary embodiment, the receiver is ableto download historical data to a physician's PC for retrospectiveanalysis by the physician. The PC communication port 76 can also be usedto interface with other medical devices, for example pacemakers,implanted analyte sensor patches, infusion devices, telemetry devices,or the like.

A user interface 78 comprises a keypad 80, speaker 82, vibrator 84,backlight 86, liquid crystal display (LCD) 88, and one or more buttons90. The components that comprise the user interface 78 provide controlsto interact with the user. The keypad 80 can allow, for example, inputof user information about himself/herself, such as mealtime, exercise,insulin administration, and reference glucose values. The speaker 82 canprovide, for example, audible signals or alerts for conditions such aspresent and/or predicted hyper- and hypoglycemic conditions. Thevibrator 84 can provide, for example, tactile signals or alerts forreasons such as described with reference to the speaker, above. Thebacklight 94 can be provided, for example, to aid the user in readingthe LCD in low light conditions. The LCD 88 can be provided, forexample, to provide the user with visual data output. In someembodiments, the LCD is a touch-activated screen. The buttons 90 canprovide for toggle, menu selection, option selection, mode selection,and reset, for example. In some alternative embodiments, a microphonecan be provided to allow for voice-activated control.

The user interface 78, which is operably connected to the microprocessor70, serves to provide data input and output for the continuous analytesensor. In some embodiments, prompts can be displayed to inform the userabout necessary maintenance procedures, such as “Calibrate Sensor” or“Replace Battery.” In some embodiments, prompts or messages can bedisplayed on the user interface to convey information to the user, suchas malfunction, outlier values, missed data transmissions, or the like.Additionally, prompts can be displayed to guide the user throughcalibration of the continuous glucose sensor, for example when to obtaina reference glucose value.

Keypad, buttons, touch-screen, and microphone are all examples ofmechanisms by which a user can input data directly into the receiver. Aserver, personal computer, personal digital assistant, insulin pump, andinsulin pen are examples of external devices that can be connected tothe receiver via PC corn port 76 to provide useful information to thereceiver. Other devices internal or external to the sensor that measureother aspects of a patient's body (for example, temperature sensor,accelerometer, heart rate monitor, oxygen monitor, or the like) can beused to provide input helpful in data processing. In one embodiment, theuser interface can prompt the patient to select an activity most closelyrelated to their present activity, which can be helpful in linking to anindividual's physiological patterns, or other data processing. Inanother embodiment, a temperature sensor and/or heart rate monitor canprovide information helpful in linking activity, metabolism, and glucoseexcursions of an individual. While a few examples of data input havebeen provided here, a variety of information can be input and can behelpful in data processing as will be understood by one skilled in theart.

Electronic Identification and Removal Noise

In addition blocking and/or diluting interfering species before they cancause noise on the sensor signal, a non-constant noise signal componentcan be electronically identified, such that the identified noisecomponent can be removed from the signal by algorithmic/mathematicalmeans, in some embodiments. For example, in a glucose sensor (e.g., asdescribed herein) that catalyzes the conversion of oxygen and glucose tohydrogen peroxide and gluconate (e.g., by GOX incorporated into themembrane system), for each glucose molecule metabolized there is anequivalent change in molecular concentration in the co-reactant O₂ andthe product H₂O₂. Consequently, one can use an electrode (for example,working electrode 16) to monitor the concentration-induced currentchange in either the co-reactant or the product (for example, H₂O₂) todetermine glucose concentration. However, if an interfering speciesexists with an oxidation or reduction potential that overlaps with theco-reactant or the product (for example, H₂O₂), then the current changedoes not accurately reflect glucose concentration. Additionally, if anoxygen deficiency exists, such that insufficient oxygen is present toreact with an analyte at the enzyme for example, then the current changesimilarly does not accurately reflect glucose concentration.

It is noted that a glucose sensor signal obtained from glucose when thebias potential is set between about +0.35V and about +0.75V issubstantially constant under standard physiologic conditions. Incontrast, a glucose sensor signal obtained from interfering species whenthe same bias potentials are set (between about +0.35V and about +0.75V)is not substantially constant under standard physiologic conditions.Current-voltage curves are known for various analytes and are availablein the literature, for example such as described by Lerner, H.; Giner,J.; Soeldner, J. S.; Colton, C. K. An implantable electrochemicalglucose sensor. Ann NY Acad Sci 1984, 428, 263-278, which isincorporated herein by reference in its entirety.

FIG. 17 is a schematic graph of current vs. voltage obtained from cyclicvoltammetry (also known as a CV-curve) for hydrogen peroxide andacetaminophen. The x-axis represents bias potential applied to anelectrochemical cell in Volts (V); the y-axis represents current outputmeasured by the working electrode of the electrochemical cell innanoAmps (nA). The schematic graph generally shows current output of anelectrochemical enzyme-based glucose sensor as the bias potential isvaried from about 0.1V to about 1.0V. Current output is shown withoutunits because it is the differential response, rather than the actualmeasurement, of signal output that is being generally taught herein. Asillustrated by the graph, acetaminophen 1722 increases the total signal1724, resulting in an inaccurate glucose measurement that issignificantly higher than the actual value.

The hydrogen peroxide curve 1720 can be obtained by exposing anelectrochemical sensor to glucose (without acetaminophen) and varyingthe bias potential from about 0.1V to about 1.0V. The graph shows theresponse of the glucose sensor to hydrogen peroxide; generally, thecurrent increases at a relatively constant rate from about 0.1V to about0.4V, after which it plateaus until about 0.6V, and then continues toincrease at a slightly slower rate.

The acetaminophen curve 1722 can be obtained by exposing anelectrochemical sensor to acetaminophen (without glucose), and varyingthe bias potential from about 0.1V to about 1.0V. The graph shows theresponse of the glucose sensor to acetaminophen; generally, theacetaminophen curve 1722 increases relatively slowly from about 0.1V toabout 0.4V, showing a minimal current output of the acetaminophen signal(at 0.4V) relative to the higher glucose signal (at 4.0V). From 0.4V to0.6V, the acetaminophen curve 1722 increases to a value at 0.6Vapproximately equal to the value of the hydrogen peroxide signal at thatsame bias potential, after which the acetaminophen curve 1722 continuesto increase at a slightly slower rate.

The total signal 1724 shows the curve that can be obtained by exposingan electrochemical sensor to glucose and acetaminophen. It isparticularly noted that at 0.6 V, acetaminophen adds significantly tothe signal output, which cause erroneously high readings of the glucoseconcentration when a presence or amount of acetaminophen is unknowinglyintroduced. In other words, the output signal of an electrochemicalsensor may not be indicative of the actual glucose concentration due tosignal interference from acetaminophen. Therefore, the preferredembodiments provide systems and methods for identifying the presence ofan interfering species and optionally deriving and analyte valuetherefrom.

In general, the preferred embodiments measure the difference between thesensor signal at low and high bias potential settings, hereinafterreferred to as the “differential measurement,” which at the minimumenables identification of signal contribution from the interferingspecies. A differential measurement that is relatively low or showssubstantial equivalence (for example, below a set threshold) identifiesa substantially glucose-only signal. In contrast, a differentialmeasurement that is relatively higher or does not show substantialequivalence (for example, above a set threshold) identifies the presenceof interfering species (for example, acetaminophen) on a glucose signal.

In some embodiments, the differential measurement can be obtained from asingle analyte-measuring device with multiple sensors. In one suchexample, the first sensor can be biased at a voltage of about +0.4V andthe second sensor can be biased at a voltage about +0.6V. The twosensors can be provided under the same membrane system or separatemembrane systems. The two sensors can share the same reference and/orcounter electrodes or can utilize separate reference and/or counterelectrodes.

In some embodiments, the differential measurement can be obtained byswitching the bias potential of a single sensor between the twomeasurement potentials. The bias potentials can be held at eachrespective setting (high and low bias settings) for as short asmilliseconds to as long as minutes or hours. Pulsed amperometricdetection (PED) is one method of quickly switching voltages, such asdescribed in Bisenberger, M.; Brauchle, C.; Hampp, N. A triple-steppotential waveform at enzyme multisensors with thick-film goldelectrodes for detection of glucose and sucrose. Sensors and Actuators1995, B, 181-189, which is incorporated herein by reference in itsentirety. In some embodiments, bias potential settings are held longenough to allow equilibration.

Additional description can be found in U.S. Pat. No. 7,081,195, toSimpson, which is incorporated by reference herein in its entirety.

Calibration Systems and Methods

As described above in the “Overview” section, continuous analyte sensorsdefine a relationship between sensor-generated measurements and areference measurement that is meaningful to a user (for example, bloodglucose in mg/dL). This defined relationship must be monitored to ensurethat the continuous analyte sensor maintains a substantially accuratecalibration and thereby continually provides meaningful values to auser. Unfortunately, both sensitivity m and baseline b of thecalibration are subject to changes that occur in vivo over time (forexample, hours to months), requiring updates to the calibration.Generally, any physical property that influences diffusion or transportof molecules through the membrane can alter the sensitivity (and/orbaseline) of the calibration. Physical properties that can alter thetransport of molecules include, but are not limited to, blockage ofsurface area due to foreign body giant cells and other barrier cells atthe biointerface, distance of capillaries from the membrane, foreignbody response/capsule, disease, tissue ingrowth, thickness of membranesystem, or the like.

In one example of a change in transport of molecules, an implantableglucose sensor is implanted in the subcutaneous space of a human, whichis at least partially covered with a biointerface membrane, such asdescribed in U.S. Patent Publication No. US-2005-0112169-A1, which isincorporated by reference herein in its entirety. Although the body'snatural response to a foreign object is to encapsulate the sensor, thearchitecture of this biointerface membrane encourages tissue ingrowthand neo-vascularization over time, providing transport of solutes (forexample, glucose and oxygen) close to the membrane that covers theelectrodes. While not wishing to be bound by theory, it is believed thatingrowth of vascularized tissue matures (changes) over time, beginningwith a short period of high solute transport during the first few daysafter implantation, continuing through a time period of significanttissue ingrowth a few days to a week or more after implantation duringwhich low solute transport to the membrane has been observed, and into amature state of vascularized tissue during which the bed of vascularizedtissue provides moderate to high solute transport, which can last formonths and even longer after implantation. In some embodiments, thismaturation process accounts for a substantial portion of the change insensitivity and/or baseline of the calibration over time due to changesin solute transport to the membrane.

Accordingly, in one aspect of the preferred embodiments, systems andmethods are provided for measuring changes in sensitivity, also referredto as changes in solute transport or biointerface changes, of an analytesensor 10 implanted in a host over a time period. Preferably, thesensitivity measurement is a signal obtained by measuring a constantanalyte other than the analyte being measured by the analyte sensor. Forexample, in a glucose sensor, a non-glucose constant analyte ismeasured, wherein the signal is measured beneath the membrane system 22on the glucose sensor 10. While not wishing to be bound by theory, it isbelieved that by monitoring the sensitivity over a time period, a changeassociated with solute transport through the membrane system 22 can bemeasured and used as an indication of a sensitivity change in theanalyte measurement. In other words, a biointerface monitor is provided,which is capable of monitoring changes in the biointerface surroundingan implantable device, thereby enabling the measurement of sensitivitychanges of an analyte sensor over time.

In some embodiments, the analyte sensor 10 is provided with an auxiliaryelectrode 18 configured as a transport-measuring electrode disposedbeneath the membrane system 22. The transport-measuring electrode can beconfigured to measure any of a number of substantially constant analytesor factors, such that a change measured by the transport-measuringelectrode can be used to indicate a change in solute (for example,glucose) transport to the membrane system 22. Some examples ofsubstantially constant analytes or factors that can be measured include,but are not limited to, oxygen, carboxylic acids (such as urea), aminoacids, hydrogen, pH, chloride, baseline, or the like. Thus, thetransport-measuring electrode provides an independent measure of changesin solute transport to the membrane, and thus sensitivity changes overtime.

In some embodiments, the transport-measuring electrode measures analytessimilar to the analyte being measured by the analyte sensor. Forexample, in some embodiments of a glucose sensor, water soluble analytesare believed to better represent the changes in sensitivity to glucoseover time than non-water soluble analytes (due to the water-solubilityof glucose), however relevant information may be ascertained from avariety of molecules. Although some specific examples are describedherein, one skilled in the art appreciates a variety of implementationsof sensitivity measurements that can be used as to qualify or quantifysolute transport through the biointerface of the analyte sensor.

In one embodiment of a glucose sensor, the transport-measuring electrodeis configured to measure urea, which is a water-soluble constant analytethat is known to react directly or indirectly at a hydrogen peroxidesensing electrode (similar to the working electrode of the glucosesensor example described in more detail above). In one exemplaryimplementation wherein urea is directly measured by thetransport-measuring electrode, the glucose sensor comprises a membranesystem as described in more detail above, however, does not include anactive interference domain or active enzyme directly above thetransport-measuring electrode, thereby allowing the urea to pass throughthe membrane system to the electroactive surface for measurementthereon. In one alternative exemplary implementation wherein urea isindirectly measured by the transport-measuring electrode, the glucosesensor comprises a membrane system as described in more detail above,and further includes an active uricase oxidase domain located directlyabove the transport-measuring electrode, thereby allowing the urea toreact at the enzyme and produce hydrogen peroxide, which can be measuredat the electroactive surface thereon.

In some embodiments, the change in sensitivity is measured by measuringa change in oxygen concentration, which can be used to provide anindependent measurement of the maturation of the biointerface, and toindicate when recalibration of the system may be advantageous. In onealternative embodiment, oxygen is measured using pulsed amperometricdetection on the glucose-measuring working electrode 16 (eliminating theneed for a separate auxiliary electrode). In another embodiment, theauxiliary electrode is configured as an oxygen-measuring electrode. Inanother embodiment, an oxygen sensor (not shown) is added to the glucosesensor, as is appreciated by one skilled in the art, eliminating theneed for an auxiliary electrode.

In some embodiments, a stability module is provided; wherein thesensitivity measurement changes can be quantified such that a co-analyteconcentration threshold is determined. A co-analyte threshold isgenerally defined as a minimum amount of co-analyte required to fullyreact with the analyte in an enzyme-based analyte sensor in anon-limiting manner. The minimum co-analyte threshold is preferablyexpressed as a ratio (for example, a glucose-to-oxygen ratio) thatdefines a concentration of co-analyte required based on a concentrationof analyte available to ensure that the enzyme reaction is limited onlyby the analyte. While not wishing to be bound by theory, it is believedthat by determining a stability of the analyte sensor based on aco-analyte threshold, the processor module can be configured tocompensate for instabilities in the glucose sensor accordingly, forexample by filtering the unstable data, suspending calibration ordisplay, or the like.

In one such embodiment, a data stream from an analyte signal ismonitored and a co-analyte threshold set, whereby the co-analytethreshold is determined based on a signal-to-noise ratio exceeding apredetermined threshold. In one embodiment, the signal-to-noisethreshold is based on measurements of variability and the sensor signalover a time period, however one skilled in the art appreciates thevariety of systems and methods available for measuring signal-to-noiseratios. Accordingly, the stability module can be configured to setdetermine the stability of the analyte sensor based on the co-analytethreshold, or the like.

In some embodiments, the stability module is configured to prohibitcalibration of the sensor responsive to the stability (or instability)of the sensor. In some embodiments, the stability module can beconfigured to trigger filtering of the glucose signal responsive to astability (or instability) of the sensor.

In some embodiments, sensitivity changes can be used to trigger arequest for one or more new reference glucose values from the host,which can be used to recalibrate the sensor. In some embodiments, thesensor is re-calibrated responsive to a sensitivity change exceeding apreselected threshold value. In some embodiments, the sensor iscalibrated repeatedly at a frequency responsive to the measuredsensitivity change. Using these techniques, patient inconvenience can beminimized because reference glucose values are generally only requestedwhen timely and appropriate (namely, when a sensitivity or baselineshift is diagnosed).

In some alternative embodiments, sensitivity changes can be used toupdate calibration. For example, the measured change in transport can beused to update the sensitivity m in the calibration equation. While notwishing to be bound by theory, it is believed that in some embodiments,the sensitivity m of the calibration of the glucose sensor issubstantially proportional to the change in solute transport measured bythe transport-measuring electrode.

It should be appreciated by one skilled in the art that in someembodiments, the implementation of sensitivity measurements of thepreferred embodiments typically necessitate an addition to, ormodification of, the existing electronics (for example, potentiostatconfiguration or settings) of the glucose sensor and/or receiver.

In some embodiments, the signal from the oxygen measuring electrode maybe digitally low-pass filtered (for example, with a passband of 0-10⁻⁵Hz, dc-24 hour cycle lengths) to remove transient fluctuations inoxygen, due to local ischemia, postural effects, periods of apnea, orthe like. Since oxygen delivery to tissues is held in tight homeostaticcontrol, this filtered oxygen signal should oscillate about a relativelyconstant. In the interstitial fluid, it is thought that the levels areabout equivalent with venous blood (40 mmHg). Once implanted, changes inthe mean of the oxygen signal (for example, >5%) may be indicative ofchange in transport through the biointerface (change in sensorsensitivity and/or baseline due to changes in solute transport) and theneed for system recalibration.

The oxygen signal may also be used in its unfiltered or a minimallyfiltered form to detect or predict oxygen deprivation-induced artifactin the glucose signal, and to control display of data to the user, orthe method of smoothing, digital filtering, or otherwise replacement ofglucose signal artifact. In some embodiments, the oxygen sensor may beimplemented in conjunction with any signal artifact detection orprediction that may be performed on the counter electrode or workingelectrode voltage signals of the electrode system. U.S. PatentPublication No. US-2005-0043598-A1, which is incorporated by referencein its entirety herein, describes some methods of signal artifactdetection and replacement that may be useful such as described herein.

Preferably, the transport-measuring electrode is located within the samelocal environment as the electrode system associated with themeasurement of glucose, such that the transport properties at thetransport-measuring electrode are substantially similar to the transportproperties at the glucose-measuring electrode.

In a second aspect the preferred embodiments, systems and methods areprovided for measuring changes baseline, namely non-glucose relatedelectroactive compounds in the host. Preferably the auxiliary workingelectrode is configured to measure the baseline of the analyte sensorover time. In some embodiments, the glucose-measuring working electrode16 is a hydrogen peroxide sensor coupled to a membrane system 22containing an active enzyme 32 located above the electrode (such asdescribed in more detail with reference to FIGS. 1 to 4, above). In someembodiments, the auxiliary working electrode 18 is another hydrogenperoxide sensor that is configured similar to the glucose-measuringworking electrode however a portion 34 of the membrane system 22 abovethe base-measuring electrode does not have active enzyme therein, suchas described in more detail with reference to FIGS. 3A and 3B. Theauxiliary working electrode 18 provides a signal substantiallycomprising the baseline signal, b, which can be (for example,electronically or digitally) subtracted from the glucose signal obtainedfrom the glucose-measuring working electrode to obtain the signalcontribution due to glucose only according to the following equation:Signal_(glucose only)=Signal_(glucose-measuring working electrode)−Signal_(baseline-measuring working electrode)

In some embodiments, electronic subtraction of the baseline signal fromthe glucose signal can be performed in the hardware of the sensor, forexample using a differential amplifier. In some alternative embodiments,digital subtraction of the baseline signal from the glucose signal canbe performed in the software or hardware of the sensor or an associatedreceiver, for example in the microprocessor.

One aspect the preferred embodiments provides for a simplifiedcalibration technique, wherein the variability of the baseline has beeneliminated (namely, subtracted). Namely, calibration of the resultantdifferential signal (Signal glucose only) can be performed with a singlematched data pair by solving the following equation:y=mx

While not wishing to be bound by theory, it is believed that bycalibrating using this simplified technique, the sensor is made lessdependent on the range of values of the matched data pairs, which can besensitive to human error in manual blood glucose measurements, forexample. Additionally, by subtracting the baseline at the sensor (ratherthan solving for the baseline b as in conventional calibration schemes),accuracy of the sensor may increase by altering control of this variable(baseline b) from the user to the sensor. It is additionally believedthat variability introduced by sensor calibration may be reduced.

In some embodiments, the glucose-measuring working electrode 16 is ahydrogen peroxide sensor coupled to a membrane system 22 containing anactive enzyme 32 located above the electrode, such as described in moredetail above; however the baseline signal is not subtracted from theglucose signal for calibration of the sensor. Rather, multiple matcheddata pairs are obtained in order to calibrate the sensor (for exampleusing y=mx+b) in a conventional manner, and the auxiliary workingelectrode 18 is used as an indicator of baseline shifts in the sensorsignal. Namely, the auxiliary working electrode 18 is monitored forchanges above a certain threshold. When a significant change isdetected, the system can trigger a request (for example, from thepatient or caregiver) for a new reference glucose value (for example,SMBG), which can be used to recalibrate the sensor. By using theauxiliary working electrode signal as an indicator of baseline shifts,recalibration requiring user interaction (namely, new reference glucosevalues) can be minimized due to timeliness and appropriateness of therequests. In some embodiments, the sensor is re-calibrated responsive toa baseline shifts exceeding a preselected threshold value. In someembodiments, the sensor is calibrated repeatedly at a frequencyresponsive to the rate-of-change of the baseline.

In yet another alternative embodiment, the electrode system of thepreferred embodiments is employed as described above, includingdetermining the differential signal of glucose less baseline current inorder to calibrate using the simplified equation (y=mx), and theauxiliary working electrode 18 is further utilized as an indicator ofbaseline shifts in the sensor signal. While not wishing to be bound bytheory, it is believed that shifts in baseline may also correlate and/orbe related to changes in the sensitivity m of the glucose signal.Consequently, a shift in baseline may be indicative of a change insensitivity m. Therefore, the auxiliary working electrode 18 ismonitored for changes above a certain threshold. When a significantchange is detected, the system can trigger a request (for example, fromthe patient or caregiver) for a new reference glucose value (forexample, SMBG), which can be used to recalibrate the sensor. By usingthe auxiliary signal as an indicator of possible sensitivity changes,recalibration requiring user interaction (new reference glucose values)can be minimized due to timeliness and appropriateness of the requests.

It is noted that infrequent new matching data pairs may be useful overtime to recalibrate the sensor because the sensitivity m of the sensormay change over time (for example, due to maturation of the biointerfacethat may increase or decrease the glucose and/or oxygen availability tothe sensor). However, the baseline shifts that have conventionallyrequired numerous and/or regular blood glucose reference measurementsfor updating calibration (for example, due to interfering species,metabolism changes, or the like) can be consistently and accuratelyeliminated using the systems and methods of the preferred embodiments,allowing reduced interaction from the patient (for example, requestingless frequent reference glucose values such as daily or even asinfrequently as monthly).

An additional advantage of the sensor of the preferred embodimentsincludes providing a method of eliminating signal effects of interferingspecies, which have conventionally been problematic in electrochemicalglucose sensors. Namely, electrochemical sensors are subject toelectrochemical reaction not only with the hydrogen peroxide (or otheranalyte to be measured), but additionally may react with otherelectroactive species that are not intentionally being measured (forexample, interfering species), which cause an increase in signalstrength due to this interference. In other words, interfering speciesare compounds with an oxidation potential that overlap with the analytebeing measured. Interfering species such as acetaminophen, ascorbate,and urate, are notorious in the art of glucose sensors for producinginaccurate signal strength when they are not properly controlled. Someglucose sensors utilize a membrane system that blocks at least someinterfering species, such as ascorbate and urate. Unfortunately, it isdifficult to find membranes that are satisfactory or reliable in use,especially in vivo, which effectively block all interferants and/orinterfering species (for example, see U.S. Pat. Nos. 4,776,944,5,356,786, 5,593,852, 5,776,324B1, and 6,356,776).

The preferred embodiments are particularly advantageous in theirinherent ability to eliminate the erroneous transient and non-transientsignal effects normally caused by interfering species. For example, ifan interferant such as acetaminophen is ingested by a host implantedwith a conventional implantable electrochemical glucose sensor (namely,one without means for eliminating acetaminophen), a transientnon-glucose related increase in signal output would occur. However, byutilizing the electrode system of the preferred embodiments, bothworking electrodes respond with substantially equivalent increasedcurrent generation due to oxidation of the acetaminophen, which would beeliminated by subtraction of the auxiliary electrode signal from theglucose-measuring electrode signal.

In summary, the system and methods of the preferred embodiments simplifythe computation processes of calibration, decreases the susceptibilityintroduced by user error in calibration, and eliminates the effects ofinterfering species. Accordingly, the sensor requires less interactionby the patient (for example, less frequent calibration), increasespatient convenience (for example, few reference glucose values), andimproves accuracy (via simple and reliable calibration).

In another aspect of the preferred embodiments, the analyte sensor isconfigured to measure any combination of changes in baseline and/or insensitivity, simultaneously and/or iteratively, using any of theabove-described systems and methods. While not wishing to be bound bytheory, the preferred embodiments provide for improved calibration ofthe sensor, increased patient convenience through less frequent patientinteraction with the sensor, less dependence on the values/range of thepaired measurements, less sensitivity to error normally found in manualreference glucose measurements, adaptation to the maturation of thebiointerface over time, elimination of erroneous signal due tonon-constant analyte-related signal so interfering species, and/orself-diagnosis of the calibration for more intelligent recalibration ofthe sensor.

Sensors Having Multiple Working Electrodes and Interferent-BlockingMembranes

In general, for a sensor configured with first and second workingelectrodes (e.g., enzyme-including and active-enzyme-lacking) the signalfrom the enzyme-lacking electrode, b, can be subtracted from theenzyme-including signal, y, and if the sensors are co-located and haveequal diffusion properties (equal sensitivities to peroxide and otherreactive species), the difference signal (y−b) is specific to theanalyte and contains no baseline. Ideally, this difference signal alsosubtracts out noise and interference (for example, acetaminophen). Inoperation, however, it is likely that the two sensors will have somewhatdifferent diffusion properties. For example, in some circumstances,differences in diffusion properties can arise from a variety of factors,such as but not limited to, small differences in manufacturing of eachelectrode, or because the membranes over the two working electrodes arenot identical (e.g., because of differences in the enzyme content orlack there of). Additionally, in some circumstances, the localenvironment adjacent to one working electrode can be substantiallydifferent from the local environment adjacent to the other workingelectrode. For example, a blood vessel (which improves interferentremoval) can be adjacent to one working electrode while the otherworking electrode can be pressed against adipose tissue (which reducesinterferent removal). Thus, in some circumstances, the difference signal(y−b) can contain some baseline, some noise and/or some interference.

Additionally, a sensor having two or more working electrodes can beaffected by sensor error, depending upon the amplitude of the baseline(e.g., non-analyte-related signal) relative to the analyte-relatedsignal. In other words, the higher the noise amplitude relative to theanalyte-related signal, the greater the error; accordingly smalldifferences in noise amplitude can induce error during calculation ofthe analyte concentration using the methods described above. Conversely,the lower noise amplitude relative to the analyte-related signal, theless the error produced, which corresponds to small changes in noiseamplitude having substantially little effect on the calibrated analyteconcentration. As an extreme example to illustrate this problem, thesignal measured by the enzyme-including electrode is equal to theanalyte-related signal plus non-analyte-related signal. If thenon-analyte-related signal is very high (e.g., 1000 counts) relative tothe analyte-related signal (e.g., 100 counts), a small change in thenon-analyte-related signal can induce substantial error when determiningthe analyte concentration. If on the other hand, the non-analyte-relatedsignal (e.g., 10 counts) is reduced relative to the analyte-relatedsignal (e.g., 100 counts), less error occurs when determining theanalyte concentration.

It is desirable to reduce noise, such that differences in enzyme andno-enzyme electrodes does not substantially not affect sensor functionand/or sensor accuracy. While not wishing to be bound by theory, it isbelieved that variability caused by baseline, noise and/or interferencesometimes observed on the difference signal (y−b) can be substantiallyeliminated by applying at least one interference domain, includingconstant and/or non-constant noise reducing mechanisms and methods asdescribed above, to a sensor having two or more working electrodes, asdescribed elsewhere herein. In other words, an interference domain cansubstantially prevent noise-causing interferents from affecting theworking electrodes, such that substantially little error is induced(e.g., when determining analyte concentration), thereby allowingimproved signal processing and greater reliability of the data.

Accordingly, in one preferred embodiment, the sensor is a sensor havingfirst and second working electrodes and a membrane system configured tosubstantially block and/or dilute interfering species (e.g., CA/CAB,Polyurethane, silicone-Pluronics blend, fluid-pocket formingconfiguration including a wound-suppressing bioagent, interferentscavengers, etc., as described elsewhere herein), wherein the firstworking electrode is enzyme-including (e.g., with GOX) and the secondworking electrode is enzyme-lacking (e.g., either no enzyme added ordeactivated/inactive enzyme added); wherein the signal on theenzyme-lacking electrode is configured to determine signal due tonon-analyte-related signal; and further wherein the signals from thefirst and second working electrodes can be processed to determine thesignal that is substantially analyte-related.

In one exemplary embodiment, the sensor is a sensor including anenzyme-including working electrode, and enzyme-lacking working electrodeand an interference domain, wherein the interference domain includes oneor more cellulosic derivatives, such as but not limited to celluloseacetate, cellulose acetate butyrate, 2-hydroxyethyl cellulose, celluloseacetate phthalate, cellulose acetate propionate, cellulose acetatetrimellitate, and the like, as well as their copolymers and terpolymerswith other cellulosic or non-cellulosic monomers, as described in thesection entitled “Interference Domain.” In addition to forming aninterference domain from only cellulose acetate(s) or only celluloseacetate butyrate(s), the interference domain can be formed fromcombinations or blends of cellulosic derivatives, such as but notlimited to cellulose acetate and cellulose acetate butyrate, orcombinations of layer(s) of cellulose acetate and layer(s) of celluloseacetate butyrate.

In another exemplary embodiment, the sensor is a sensor including anenzyme-including working electrode, and enzyme-lacking working electrodeand an interference domain, wherein the interference domain includes ablend of a silicone material and a hydrophilic polymer (e.g., ahydrophilic-hydrophobic polymer, such as but not limited to a PLURONIC®polymer). In a preferred embodiment, the blend is a substantial blend.For example, the ratio of silicone polymer to hydrophilic polymer, aswell as the polymeric compositions, can be selected such that a layerconstructed from the material has interference characteristics thatinhibit transport of one or more interfering species (described above)through the layer.

In yet another exemplary embodiment, the sensor is a sensor including anenzyme-including working electrode, and enzyme-lacking working electrodeand an interference domain, wherein the interference domain includespolyurethanes, polymers having pendant ionic groups, and/or polymershaving controlled pore size. In a further embodiment, the interferencedomain includes a thin, hydrophobic membrane that is non-swellable andrestricts diffusion of low molecular weight species. Additionally, theinterference domain is permeable to relatively low molecular weightsubstances, such as hydrogen peroxide, but restricts the passage ofhigher molecular weight substances, including glucose and ascorbic acid.

In still another exemplary embodiment, the sensor is a sensor includingan enzyme-including working electrode, and enzyme-lacking workingelectrode and an interferent blocking membrane system, wherein themembrane system includes one or more interferent-blockingconfigurations, such as but not limited to an interferent-blockinginterference layer, an interferent-blocking resistance domain, a porousmembrane configured to suppress interferent build-up, aninterferent-eliminating auxiliary electrode, an interferent-scavengerand the like.

EXAMPLES Example 1 Dual-Electrode Sensor with Coiled Reference Electrode

Dual-electrode sensors (having a configuration similar to the embodimentshown in FIG. 9B) were constructed from two platinum wires, each coatedwith non-conductive material/insulator. Exposed electroactive windowswere cut into the wires by removing a portion thereof. The platinumwires were laid next to each other such that the windows are offset(e.g., separated by a diffusion barrier). The bundle was then placedinto a winding machine & silver wire was wrapped around the platinumelectrodes. The silver wire was then chloridized to produce asilver/silver chloride reference electrode. The sensor was trimmed tolength, and a glucose oxidase enzyme solution applied to both windows(e.g., enzyme applied to both sensors). To deactivate the enzyme in onewindow (e.g., window 904 a, FIG. 9B) the window was dipped intodimethylacetamide (DMAC) and rinsed. After the sensor was dried, aresistance layer was sprayed onto the sensor and dried.

FIG. 12 shows the results from one experiment, comparing the signalsfrom the two electrodes of the dual-electrode sensor having a coiledsilver/silver chloride wire reference electrode described above. The“Plus GOx” electrode included active GOx in its window. The “No GOx”electrode included DMAC-inactivated GOx in its window. To test, thesensor was incubated in room temperature phosphate buffered saline (PBS)for 30 minutes. During this time, the signals from the two electrodeswere substantially equivalent. Then the sensor was moved to a 40-mg/dlsolution of glucose in PBS. This increase in glucose concentrationresulting in an expected rise in signal from the “Plus GOx” electrodebut no significant increase in signal from the “No GOx” electrode. Thesensor was then moved to a 200-mg/dl solution of glucose in PBS. Again,the “Plus GOx” electrode responded with a characteristic signal increasewhile no increase in signal was observed for the “No GOx” electrode. Thesensor was then moved to a 400-mg/dl solution of glucose in PBS. The“Plus GOx” electrode signal increased to about 5000 counts while noincrease in signal was observed for the “No GOx” electrode. As a finaltest, the sensor was moved to a solution of 400 mg/dl glucose plus 0.22mM acetaminophen (a known interferant) in PBS. Both electrodes recordedsimilarly dramatic increases in signal (raw counts). These data indicatethat the “No GOx” electrode is measuring sensor background (e.g., noise)that is substantially related to non-glucose factors.

Example 2 Dual-Electrode Sensor with X-Shaped Reference Electrode

This sensor was constructed similarly to the sensor of Example 1, exceptthat the configuration was similar to the embodiment shown in FIG. 7J.Two platinum electrode wires were dipped into non-conductive materialand then electroactive windows formed by removing portions of thenonconductive material. The two wires were then bundled with an X-shapedsilver reference electrode therebetween. An additional layer ofnon-conductive material held the bundle together.

FIG. 13 shows the results from one experiment, comparing the signalsfrom the two electrodes of a dual-electrode sensor having an X-shapedreference electrode. The “Plus GOx” electrode has active GOx in itswindow. The “No GOx” electrode has DMAC-inactivated GOx in its window.The sensor was tested as was described for Experiment 1, above. Signalfrom the two electrodes were substantially equivalent until the sensorwas transferred to the 40-mg/dl glucose solution. As this point, the“Plus GOx” electrode signal increased but the “No GOx” electrode signaldid not. Similar increases were observed in the “Plus GOx” signal whenthe sensor was moved consecutively to 200-mg/dl and 400-mg/dl glucosesolution, but still not increase in the “No GOx” signal was observed.When sensor was moved to a 400-mg/dl glucose solution containing 0.22 mMacetaminophen, both electrodes recorded a similar increase in signal(raw counts). These data indicate that the “No GOx” electrode measuressensor background (e.g., noise) signal that is substantially related tonon-glucose factors.

Example 3 Dual-Electrode Challenge with Hydrogen Peroxide, Glucose, andAcetaminophen

A dual-electrode sensor was assembled similarly to the sensor of Example1, with a bundled configuration similar to that shown in FIG. 7C (twoplatinum working electrodes and one silver/silver chloride referenceelectrode, not twisted). The electroactive windows were staggered by0.085 inches, to create a diffusion barrier.

FIG. 14 shows the experimental results. The Y-axis shows the glucosesignal (volts) and the X-axis shows time. The “Enzyme” electrodeincluded active GOx. The “No Enzyme” electrode did not include activeGOx. The “Enzyme minus No Enzyme” represents a simple subtraction of the“Enzyme” minus the “NO Enzyme.” The “Enzyme” electrode measures theglucose-related signal and the non-glucose-related signal. The “NoEnzyme” electrode measures only the non-glucose-related signal. The“Enzyme minus No Enzyme” graph illustrates the portion of the “Enzyme”signal related to only the glucose-related signal.

The sensor was challenged with increasing concentrations of hydrogenperoxide in PBS. As expected, both the “Enzyme” and “No Enzyme”electrodes responded substantially the same with increases in signalcorresponding increased in H₂O₂ concentration (˜50 μM, 100 μM and 250 μMH₂O₂). When the “No Enzyme” signal was subtracted from the “Enzyme”signal, the graph indicated that the signal was not related to glucoseconcentration.

The sensor was challenged with increasing concentrations of glucose (˜20mg/dl, 200 mg/dl, 400 mg/dl) in PBS. As glucose concentration increased,the “Enzyme” electrode registered a corresponding increase in signal. Incontrast, the “No Enzyme” electrode did not record an increase insignal. Subtracting the “No Enzyme” signal from the “Enzyme” signalshows a step-wise increase in signal related to only glucoseconcentration.

The sensor was challenged with the addition of acetaminophen (˜0.22 mM)to the highest glucose concentration. Acetaminophen is known to be aninterferent (e.g., produces non-constant noise) of the sensors built asdescribed above, e.g., due to a lack of acetaminophen-blocking membraneand/or mechanism formed thereon or provided therewith. Both the “Enzyme”and “No Enzyme” electrodes showed a substantial increase in signal. The“Enzyme minus No Enzyme” graph substantially shows the portion of thesignal that was related to glucose concentration.

From these data, it is believed that a dual-electrode system can be usedto determine the analyte-only portion of the signal.

Example 4 IV Dual-Electrode Sensor in Dogs

An intravascular dual-electrode sensor was built substantially asdescribed in co-pending U.S. patent application Ser. No. 11/543,396filed on Oct. 4, 2006, and entitled “ANALYTE SENSOR.” Namely, the sensorwas built by providing two platinum wires (e.g., dual workingelectrodes) and vapor-depositing the platinum wires with Parylene toform an insulating coating. A portion of the insulation on each wire wasremoved to expose the electroactive surfaces (e.g., 904 a and 904 b).The wires were bundled such that the windows were offset to provide adiffusion barrier, as described herein, cut to the desired length, toform an “assembly.” A silver/silver chloride reference electrode wasdisposed remotely from the working electrodes (e.g., coiled inside thesensor's fluid connector).

An electrode domain was formed over the electroactive surface areas ofthe working electrodes by dip coating the assembly in an electrodesolution (comprising BAYHYDROL® 123 with PVP and added EDC) and drying.

An enzyme domain was formed over the electrode domain by subsequentlydip coating the assembly in an enzyme domain solution (BAYHYDROL 140AQmixed with glucose oxidase and glutaraldehyde) and drying. This dipcoating process was repeated once more to form an enzyme domain havingtwo layers and subsequently drying. Next an enzyme solution containingactive GOx was applied to one window; and an enzyme solution withoutenzyme (e.g., No GOx) was applied to the other window.

A resistance domain was formed over the enzyme domain by subsequentlyspray coating the assembly with a resistance domain solution(Chronothane H and Chronothane 1020) and drying.

After the sensor was constructed, it was placed in a protective sheathand then threaded through and attached to a fluid coupler, as describedin co-pending U.S. patent application Ser. No. 11/543,396 filed on Oct.4, 2006 and entitled “ANALYTE SENSOR.” Prior to use, the sensors weresterilized using electron beam radiation.

The forelimb of an anesthetized dog (2 years old, ˜40 pounds) was cutdown to the femoral artery and vein. An arterio-venous shunt was placedfrom the femoral artery to the femoral vein using 14 gauge catheters and⅛-inch IV tubing. A pressurized arterial fluid line was connected to thesensor systems at all times. The test sensor system included a 20gauge×1.25-inch catheter and took measurements every 30 seconds. Thecatheter was aseptically inserted into the shunt, followed by insertionof the sensor into the catheter. As controls, the dog's glucose waschecked with an SMBG, as well as removing blood samples and measuringthe glucose concentration with a Hemocue.

FIG. 15 shows the experimental results. Glucose test data (counts) isshown on the left-hand Y-axis, glucose concentration for the controls(SMBG and Hemocue) are shown on the right-hand y-axis and time is shownon the X-axis. Each time interval on the X-axis represents 29-minutes(e.g., 12:11 to 12:40 equals 29 minutes). An acetaminophen challenge isshown as a vertical line on the graph.

The term “Plus GOx” refers to the signal from the electrode coated withactive GOx., which represents signal due to both the glucoseconcentration and non-glucose-related electroactive compounds asdescribed elsewhere herein (e.g., glucose signal and background signal,which includes both constant and non-constant noise). “No GOx” is signalfrom the electrode lacking GOx, which represents non-glucose relatedsignal (e.g., background signal, which includes both constant andnon-constant noise). The “Glucose Only” signal (e.g., related only toglucose concentration) is determined during data analysis (e.g., bysensor electronics). In this experiment, the “Glucose Only” signal wasdetermined by a subtraction of the “No GOx” signal from the “Plus GOx”signal.

During the experiment, the “No GOx” signal (thin line) substantiallyparalleled the “Plus GOx” signal (medium line). The “Glucose Only”signal substantially paralleled the control tests (SMBG/Hemocue).

Acetaminophen is known to be an interferent (e.g., produces non-constantnoise) of the sensors built as described above, e.g., due to a lack ofacetaminophen-blocking membrane and/or mechanism formed thereon orprovided therewith. The SMBG or Hemocue devices utilized in thisexperiment, however, do include mechanisms that substantially blockacetaminophen from the signal (see FIG. 15). When the dog was challengedwith acetaminophen, the signals from both working electrodes (“Plus GOx”and “No GOx”) increased in a substantially similar manner. When the“Glucose Only” signal was determined, it substantially paralleled thesignals of the control devices and was of a substantially similarmagnitude.

From these experimental results, it is believed that an indwelling,dual-electrode glucose sensor system (as described herein) in contactwith the circulatory system can provide substantially continuous glucosedata that can be used to calculate a glucose concentration that is freefrom background components (e.g., constant and non-constant noise), in aclinical setting.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. Pat. Nos.4,994,167; 4,757,022; 6,001,067; 6,741,877; 6,702,857; 6,558,321;6,931,327; 6,862,465; 7,074,307; 7,081,195; 7,108,778; and 7,110,803.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. PatentPublication No. US-2005-0176136-A1; U.S. Patent Publication No.US-2005-0251083-A1; U.S. Patent Publication No. US-2005-0143635-A1; U.S.Patent Publication No. US-2005-0181012-A1; U.S. Patent Publication No.US-2005-0177036-A1; U.S. Patent Publication No. US-2005-0124873-A1; U.S.Patent Publication No. US-2005-0115832-A1; U.S. Patent Publication No.US-2005-0245799-A1; U.S. Patent Publication No. US-2005-0245795-A1; U.S.Patent Publication No. US-2005-0242479-A1; U.S. Patent Publication No.US-2005-0182451-A1; U.S. Patent Publication No. US-2005-0056552-A1; U.S.Patent Publication No. US-2005-0192557-A1; U.S. Patent Publication No.US-2005-0154271-A1; U.S. Patent Publication No. US-2004-0199059-A1; U.S.Patent Publication No. US-2005-0054909-A1; U.S. Patent Publication No.US-2005-0112169-A1; U.S. Patent Publication No. US-2005-0051427-A1; U.S.Patent Publication No. US-2003-0032874-A1; U.S. Patent Publication No.US-2005-0103625-A1; U.S. Patent Publication No. US-2005-0203360-A1; U.S.Patent Publication No. US-2005-0090607-A1; U.S. Patent Publication No.US-2005-0187720-A1; U.S. Patent Publication No. US-2005-0161346-A1; U.S.Patent Publication No. US-2006-0015020-A1; U.S. Patent Publication No.US-2005-0043598-A1; U.S. Patent Publication No. US-2003-0217966-A1; U.S.Patent Publication No. US-2005-0033132-A1; U.S. Patent Publication No.US-2005-0031689-A1; U.S. Patent Publication No. US-2004-0186362-A1; U.S.Patent Publication No. US-2005-0027463-A1; U.S. Patent Publication No.US-2005-0027181-A1; U.S. Patent Publication No. US-2005-0027180-A1; U.S.Patent Publication No. US-2006-0020187-A1; U.S. Patent Publication No.US-2006-0036142-A1; U.S. Patent Publication No. US-2006-0020192-A1; U.S.Patent Publication No. US-2006-0036143-A1; U.S. Patent Publication No.US-2006-0036140-A1; U.S. Patent Publication No. US-2006-0019327-A1; U.S.Patent Publication No. US-2006-0020186-A1; U.S. Patent Publication No.US-2006-0020189-A1; U.S. Patent Publication No. US-2006-0036139-A1; U.S.Patent Publication No. US-2006-0020191-A1; U.S. Patent Publication No.US-2006-0020188-A1; U.S. Patent Publication No. US-2006-0036141-A1; U.S.Patent Publication No. US-2006-0020190-A1; U.S. Patent Publication No.US-2006-0036145-A1; U.S. Patent Publication No. US-2006-0036144-A1; U.S.Patent Publication No. US-2006-0016700-A1; U.S. Patent Publication No.US-2006-0142651-A1; U.S. Patent Publication No. US-2006-0086624-A1; U.S.Patent Publication No. US-2006-0068208-A1; U.S. Patent Publication No.US-2006-0040402-A1; U.S. Patent Publication No. US-2006-0036142-A1; U.S.Patent Publication No. US-2006-0036141-A1; U.S. Patent Publication No.US-2006-0036143-A1; U.S. Patent Publication No. US-2006-0036140-A1; U.S.Patent Publication No. US-2006-0036139-A1; U.S. Patent Publication No.US-2006-0142651-A1; U.S. Patent Publication No. US-2006-0036145-A1; U.S.Patent Publication No. US-2006-0036144-A1; U.S. Patent Publication No.US-2006-0200022-A1; U.S. Patent Publication No. US-2006-0198864-A1; U.S.Patent Publication No. US-2006-0200019-A1; U.S. Patent Publication No.US-2006-0189856-A1; U.S. Patent Publication No. US-2006-0200020-A1; U.S.Patent Publication No. US-2006-0200970-A1; U.S. Patent Publication No.US-2006-0183984-A1; U.S. Patent Publication No. US-2006-0183985-A1; U.S.Patent Publication No. US-2006-0195029-A1; U.S. Patent Publication No.US-2006-0229512-A1; U.S. Patent Publication No. US-2006-0222566-A1; U.S.Patent Publication No. US-2007-0032706-A1; U.S. Patent Publication No.US-2007-0016381-A1; U.S. Patent Publication No. US-2007-0027370-A1; U.S.Patent Publication No. US-2007-0027384-A1; U.S. Patent Publication No.US-2007-0032717-A1; and U.S. Patent Publication No. US-2007-0032718 A1.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. applicationSer. No. 09/447,227 filed Nov. 22, 1999 and entitled “DEVICE AND METHODFOR DETERMINING ANALYTE LEVELS”; U.S. application Ser. No. 11/515,342filed Sep. 1, 2006 and entitled “SYSTEMS AND METHODS FOR PROCESSINGANALYTE SENSOR DATA”; U.S. application Ser. No. 11/654,135 filed Jan.17, 2007 and entitled “POROUS MEMBRANES FOR USE WITH IMPLANTABLEDEVICES”; U.S. application Ser. No. 11/675,063 filed Feb. 14, 2007 andentitled “ANALYTE SENSOR”; U.S. application Ser. No. 11/543,734 filedOct. 4, 2006 and entitled “DUAL ELECTRODE SYSTEM FOR A CONTINUOUSANALYTE SENSOR”; U.S. application Ser. No. 11/654,140 filed Jan. 17,2007 and entitled “MEMBRANES FOR AN ANALYTE SENSOR”; U.S. applicationSer. No. 11/654,327 filed Jan. 17, 2007 and entitled “MEMBRANES FOR ANANALYTE SENSOR”;U.S. application Ser. No. 11/543,396 filed Oct. 4, 2006and entitled “ANALYTE SENSOR”; U.S. application Ser. No. 11/543,490filed Oct. 4, 2006 and entitled “ANALYTE SENSOR”; U.S. application Ser.No. 11/543,404 filed Oct. 4, 2006 and entitled “ANALYTE SENSOR”; U.S.application Ser. No. 11/681,145 filed Mar. 1, 2007 and entitled “ANALYTESENSOR”; and U.S. application Ser. No. 11/690,752 filed Mar. 23, 2007and entitled “TRANSCUTANEOUS ANALYTE SENSOR”.

All references cited herein, including but not limited to published andunpublished applications, patents, and literature references, areincorporated herein by reference in their entirety and are hereby made apart of this specification. To the extent publications and patents orpatent applications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention.

1. A continuous glucose monitoring system configured for measuringglucose concentration in a host, the system comprising: a continuousglucose sensor comprising a first working electrode disposed beneath anactive enzymatic portion of a membrane system and configured to generatea first signal, and a second working electrode disposed beneath aninactive-enzymatic or a non-enzymatic portion of the membrane system andconfigured to generate a second signal, wherein the membrane system isconfigured to substantially reduce interfering species from reaching thefirst working electrode and the second working electrode; andelectronics configured to process the first signal and the second signalto produce a glucose signal that is substantially without signalcontribution from interfering species, and to monitor the second signalfor a change in amplitude above a threshold.
 2. The system of claim 1,wherein the membrane system comprises an interference domain.
 3. Amethod for providing a substantially noise-free glucose signal for aglucose sensor implanted in a host, the method comprising: providing aglucose sensor, the glucose sensor comprising a first working electrodedisposed beneath an active enzymatic portion of a membrane system, and asecond working electrode disposed beneath an inactive-enzymatic or anon-enzymatic portion of the membrane system, wherein the membranesystem is configured to substantially reduce one or more interferingspecies from reaching the first working electrode and the second workingelectrode; generating a first signal associated with the first workingelectrode; generating a second signal associated with the second workingelectrode; processing the first signal and the second signal to producea glucose signal that is substantially without signal contribution frominterfering species; and monitoring the second signal for a change inamplitude above a threshold.
 4. The method of claim 3, furthercomprising requesting an external reference value when the change isabove the threshold.
 5. The method of claim 3, further comprising notcalibrating the glucose signal when the change is above the threshold.6. The method of claim 3, further comprising determining an instabilityof the glucose signal when the change is above the threshold.
 7. Thesystem of claim 6, further comprising controlling a display of theglucose signal based on the determined instability.